I don't feel that answer (D) is a correct inference. It was more of a direct statement within the passage than something to be extrapolated. Furthermore, the subject of (D) is "people" which is overly broad.
While Choice D is very similar to the sentence in the passage, it is not a direct re-statement so it does require inference. The passage says "for those who volunteer too many hours, role theory suggests they may experience role strain." Therefore we can reasonably infer that volunteering "too often" will lead to role strain. "People" is not overly broad because the passage does not limit its discussion of role strain to any specific population, it just says "those who volunteer."