Assuming that the new drug is approved, it will remain to be seen whet

Official Explanation

Creating a filter: We read the sentence and, drifting to the answer choices, see a 3 versus 2 standoff of "whether" against "if." This is a yes-or-no question, not a condition, so "whether" is correct and "if" is incorrect. We'll filter for choices that use "whether."

Applying the filter: our filter leaves us with (A) through (C).

Finding objective defects: choice (B) introduces an ambiguous pronoun "they," which seems to indicate that veterans are administering the drug, contrary to intended meaning. Choices (A) and (C) are similar. Which conveys the intended meaning? We can see that (A) is imprecise and (C) is precise. Contrary to the intended meaning, (A) makes it sound as if we might be wondering whether the new drug will have an effect before it's administered, or during its administration. The administration is meant to be the precondition for the intended effect. The word "once" and the verb tense "has been" both do that accurately. Here the present perfect verb "has been administered" is fitting, because we want to convey that the administration occurs right before the effect will or will not be seen.

The correct answer is (C).