Hi Alex! Thanks so much for your great question
Yes, answer choice (D) is correct because if we take the contrapositive of the conditional, we get the following: if milk from BST cows is not successfully screened for high levels of antibiotics, it cannot be presumed safe for humans. The stimulus tells us that high levels of antibiotics given to BST-treated cows can be harmful to humans, as well as that screening is performed on the milk of both treated and untreated cows. So if we don't screen the milk from these cows to determine if there are high levels of antibiotics, then we cannot know whether or not the milk is safe for humans since these treated cows are more likely to be given antibiotics. In other words, answer choice (D) properly combines facts in the stimulus to draw a reasonably supported inference. The tricky thing about answer choice (D) is the fact that it's presented as a conditional statement!
As you noted, the rest of the answer choices are not supported by the stimulus:
Answer choice (A) is incorrect because the author of the stimulus does not suggest whether or not he thinks the consumer groups are justified in opposing the use of BST.
Answer choice (B) is incorrect because the author never compares the relative safety of milk from treated vs. untreated cows. All we know is that the milk from both groups of cows is screened, but this doesn't tell us anything about their relative safety.
Answer choice (C) is incorrect because the stimulus does not discuss whether there are advantages to the use of BST.
Answer choice (E) is incorrect because we do not know that this is the only threat from drinking the milk of treated cows; all we know is that this is one possible danger to drinking the milk.
To sum up, process of elimination can be extremely helpful on Most Strongly Supported questions! If you can eliminate all four wrong answer choices confidently because they do not have any support in the stimulus, you can select the remaining answer choice once you make sure that there is information presented in the stimulus to support it.
I hope this helps, and let me know if you have any other questions!