Official Solution

Usually, when you find pronoun decision points in a sentence correction problem, you should start by assessing those. However, in this problem that decision point is quite tricky! When assessing “it” versus “them,” you might think that the pronoun should be plural, as it seems to be referring to calories. But if it is singular, it could also refer to food and this too is logical (in fact you don’t really “store calories as fat” but you do “store food as fat”). Since both seem reasonable, look for other decision points. One important consideration is that you cannot mix and match the pronouns – either both need to be singular or both need to be plural. That allows you to definitely eliminate (C) and (E) as they incorrectly mix and match a singular and plural pronoun. The other critical decision point between (A), (B), and (D) is the “burning” versus “to burn” or “burn.” Checking parallelism, you see that this expression needs to be: “STORING more of it as fat rather than BURNING it for energy.” Comparisons such as this require strict parallelism and (A) and (B) both clearly violate that. While you might be a bit perplexed by the singular pronoun choice in (D), it is the only one that avoids concrete errors of agreement and parallelism. The correct answer is (D).


Taken from this NY Times Article
nytimes.com/2018/01/01/science/food-fiber-microbiome-inflammation.html