You are looking for something that explains how or why a more active premature baby would gain more weight than a less active premature baby.

(A) Because the prompt says that food intake is the same for all the babies, level of hunger is irrelevant, as increased hunger does not lead to increased food intake.

(B) This does not seem to have a link to weight gain. That having been said, if you were to see this answer choice on the test, you might not want to throw it out right away, as often an answer choice that initially seems to not have any connection to the question turns out to be the correct answer.

(C) This explains the weight gain. Active babies may not take in more food than less active babies do, and active babies may burn more calories, BUT because the intestines of more active babies are such that more active babies absorb more food than less active babies do, effectively the more active babies are getting more nutrition. So the difference in absorption explains the difference in weight gain.

(D) This does not explain why massage, and the resulting activity, are correlated with increased weight.

(E) The prompt and question discuss only premature babies, and what this answer choice says does not explain anything having to do with differences in the weights of premature babies who have different levels of activity.

The correct answer is C.