I feel like an anti-LSAT writers mindset is quite useless, if anything it probably hinders progress. It's important to understand how thought out each question has to be in order to make it on a real test. It's unlikely that the writers are mistaken in any given circumstance.

Within the stimulus, the author claims that insulin's purpose when excess carbs are present is "to store the resulting by-products as fat". So the way I interpreted the last sentence is that carbs are either A) used for energy when the body needs it or B) stored as fat when excess carbs are present. Answer choice D refers to the causal relationship between eating high simple carbs and gaining fat, the same relationship that is stated in the stimulus.