You are answering a USMLE-style medical question. Question: A 4-year-old boy is brought to his primary care physician for a physical prior to beginning a preschool program. He is found to be delayed in reaching normal childhood cognitive and social milestones, which is consistent with observations at previous check-ups. Physically, he is also found to be significantly overweight with hypogonadism. Notably, the boy was limp when he was born and did not have a normal motor response to stimuli. Elevated levels of a gastrointestinal hormone are associated with the cause of this patient's weight gain. The hormone that is most responsible for causing the weight gain is produced in which of the following cell types? Options: A) D cells B) G cells C) P/D1 cells D) L cells Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.