Emily,
You're onto something here! Let's take a look:
The stimulus begins with the author’s conclusion that we will not be able to determine the presence of sentient beings outside our solar system unless some of them are at least as smart as humans. Although the stimulus begins with the familiar term “if,” the first sentence could be more simply restructured as follows:
We will not be able to determine the existence of other sentient beings (even if they do actually exist) unless some of them are at least as intelligent as humans.
We can diagram this statement by applying the Unless Equation: the phrase modified by “unless” becomes the necessary condition, whereas the remainder is negated and becomes the sufficient condition. Thus, the first sentence can be diagrammed as follows:
S N
Conclusion: Determine Exist 
As Intelligent
Even though there are no conclusion indicators to help us identify the conclusion, note that the second sentence contains two premises that support the observation in the first sentence: first, we will not be able to send spacecraft outside our solar system in the near future, and second, any sentient being able to communicate with us would need to be at least as intelligent as we are.
- Premise (1): Send Spacecraft
Premise (2): SB Communicate As Intelligent
In other words, if we want to determine whether sentient beings exist outside our solar system, one approach might be to send a spacecraft. Without spacecraft as an option, however, the author asserts that we would have to rely on these beings’ ability to communicate with us. This, in turn, would require that such sentient beings be at least as intelligent as humans.
The question stem asks us to identify a statement that, if assumed, would enable to the conclusion to be properly drawn. As with most Justify questions, there is a logical gap between the premises and the conclusion. To prove the conclusion, we must establish that there are only two ways to determine the existence of sentient begins outside our solar system: either we send a spacecraft outside our solar system, or the sentient themselves must be able to communicate with us. That way, if one of the two options is ruled out, the only other way to determine the existence of sentient beings would be to pursue the other option:
- Justify Formula: CANNOT Send Spacecraft SB Communicate
Note that the relationship between the two options contains a negative sufficient condition—if one cannot occur, the other must occur. The contrapositive would serve the same function: if the sentient beings cannot communicate with us, then the only other option for determining their existence would be to send a spacecraft to its planet:
- Justify (Contrap.): SB CANNOT Communicate Send Spacecraft
Since the contrapositive is logically identical to our prephrase, answer choice (D) is correct.
Another way to approach this Justify question would be as follows:
- (1) Elements that appear in the premises but not the conclusion usually appear in the correct answer. Although these premise elements do not have to appear in the correct answer, they often do because they represent a convenient linking point. In this argument, “Send Spacecraft” appears in the premises but not in the conclusion. Therefore, it is highly likely that it would appear in the correct answer. Its absence from answer choices (A), (B), and (E) helps eliminate them.
(2) Elements that are common to the conclusion and at least one premise normally do not appear in the correct answer, because there is a bridge already established that justifies the presence of that element in the conclusion. Here, the element of “Determine Exist” is common to both the premises and the conclusion and need not be present in the correct answer. Its presence in answer choices (B), (C), and (E) helps eliminate them.
Applying these two rules of solving Justify questions mechanistically leaves answer choice (D) as the only possible contender.
Answer choice (A): Because the conclusion focuses on whether sentient beings exist outside our solar system, the fact that they do not exist on planets inside our solar system is irrelevant. Even if there were no sentient beings on planets in our solar system, that alone does not prove that there are only two ways of determining their existence.
A savvy test taker would eliminate this answer choice immediately because it does not contain the element of “Send Spacecraft.” Since this element appears in the premises but not in the conclusion, we would expect it to appear in the correct answer.
Answer choice (B): Even if the sentient beings would want to communicate with us, that does not prove that we would have to rely on their ability to communicate with us. Because the possibility remains that there is a third way to determine their existence, the conclusion is not fully justified.
To some test takers, this may seem like an assumption answer (which would still make it incorrect, as we are not answering an Assumption question). However, applying the Assumption Negation technique reveals that this is not an assumption: even if the sentient beings did not want to communicate with us, that would not weaken the conclusion that determining their existence requires that these beings be at least as intelligent as we are. Granted, if they refused to communicate with us, our job of determining their existence would be made considerably more difficult, but this is irrelevant to the conclusion of the argument. Therefore, answer choice (B) is neither sufficient, nor necessary, for the conclusion to be true.
Answer choice (C): Since the author already established that sending a spacecraft to planets outside our solar system is impossible in the near future, this answer choice merely restates a premise in the argument. If you found this answer choice attractive, you probably mistook a Justify question stem for a Must Be True.
Answer choice (D): This is the correct answer, as it is the contrapositive of the Justify Formula outlined above.
In other words, without the option of sending a spacecraft outside out solar system, the only other way to determine the existence of sentient beings is for them to communicate with us. Since their ability to communicate with us requires intelligence, it logically follows that determining the existence of sentient beings outside our solar system requires that such beings be at least as intelligent as humans.
Note that the Justify Formula could have been phrased in a number of ways:
If we cannot send a spacecraft outside our solar system in an effort to determine the existence of sentient beings, we must rely on these beings’ ability to communicate with us.
To determine if there are sentient beings on other planets outside our solar system, we must either send a spacecraft there, or the sentient beings themselves must be able to communicate with us.
Answer choice (E): This is a Mistaken Reversal of the second premise in the argument. The author asserts that the sentient beings’ ability to communicate with us depends on their intelligence. If the reverse were also true (i.e. that any sentient being that is at least as intelligent as humans would be capable of communicating with us), this would not prove the conclusion. The possibility remains that there is a third way to determine the existence of sentient beings outside our solar system.
A savvy smart test taker would eliminate this answer choice immediately because it does not contain the element of “Send Spacecraft.” Since this element appeared in the premises but not in the conclusion, we would expect it to appear in the correct answer.
Hope this helps!
