Hi Joshua! Happy to explain why E is not the correct answer
On Main Point questions, we want to find the answer choice that accurately summarizes the conclusion of the argument/what the author's point of view is. Any answer choice that restates a premise or an intermediate/subsidiary conclusion, for example, is wrong.
In this argument, the conclusion is that research efforts to make Mars's climate inhabitable are justified. The author gives several premises to support this conclusion, and Adam's post earlier in this thread does a great job describing how the argument proceeds.
Answer choice (D) is a restatement of the main conclusion of the stimulus, making it the correct answer choice.
Answer choice (E), however, restates the last sentence of the stimulus, which is a premise offered to support the main conclusion. Notice that the stimulus does not offer any evidence to support the claim that understanding how to change Mars's atmosphere may help us understand Earth's atmospheric changes. Instead, this sentence gives an additional reason (beyond the fact that we can probably make the climate of Mars inhabitable) for why research efforts are justifiable. The word "Besides" in this sentence also gives us a hint that this sentence is likely a premise.
In general, when approaching Main Point questions, be sure to mentally re-order the sentences in the stimulus such that the order reflects how the premises lead to the conclusion.
I hope this helps!