This is a Weaken question. First, let's examine the stimulus.
The newspaper article presents an argument, the conclusion of which is "People who take vitamin C supplements tend to be healthier than average." The premise offered in support of this conclusion refers to a study that links high doses of vitamin C to significantly lower-than-average risk of heart disease.
Great. See how you have a premise about one aspect of health (risk of heart disease) but a conclusion about overall health? That's a classic LSAT trick--and a weak argument.
Answer choice (D) expresses this problem: if it is true that high doses of vitamin C tend to reduce one's resistance to certain common infectious diseases, then the people consuming high doses of vitamin C may have healthier hearts (per the premise in the stimulus) but be more prone to, e.g., the flu, and thus NOT be healthier than average (weakening the conclusion).
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