The correct option is D After sentence (2)
The paragraph in question here, deals entirely with the future demand and role of dietitians and nutritionists in the U.S. Sentence 4 placed where it is now, does not clearly convey the reference of 'their patients'- it seems like a general reference which does not fit with the paragraph (seems out of context). When it [sentence 4] is placed after sentence 2, sentence 4 makes more sense and we know that 'patients' refers to the 'baby boomer generation'. So, only option C fits in this context. C is our answer.