(A) is a clever trap answer: it wants to trick us into assuming that we need a certain amount of muscle and bone tissue to be healthy, and that weight gain lets us maintain that amount. However, those are both unwarranted assumptions, and without them, (A) doesn't work.

The problem is that (A) doesn't indicate if this change has any health effects, let alone negative health effects. The stimulus also doesn't go into this at all. That's why (A) is a trap—it doesn't have enough information to actually address the contradiction.

To resolve the contradiction, we need to explain why participants who gained weight lived longer. (B) gives us the opposite: a mechanism for how people who lose weight a particular way might live longer. That doesn't help to resolve our stimulus.

Now we know there's a particular group of people who are less likely to gain weight, but have shorter lifespans. That's exactly the kind of alternative explanation we're looking for. As far as we know, the study didn't exclude smokers—so the finding that weight gain correlated with longer lifespan could be explained by non-weight-gaining smokers have a shorter lifespan.

(C) isn't a perfect explanation: we don't know for sure that the study included smokers at all, let alone enough to affect the results. However, it takes us closer to explaining the confusing results, which means it helps to resolve the contradiction. That's all we need.

In other words, cutting calories can keep you healthier for longer. This doesn't resolve anything, it only deepens our confusion. We need to explain why weight gain is associated with longer lifespans, not how a weight loss strategy can improve health.

(E) is similar to (B) and (D) in that it only makes the contradiction stronger. To explain the contradictory study results, we need to show why weight gain was associated with longer lifespan. Emphasizing a correlation between weight gain and health problems does the opposite.