The best answer is D. The second paragraph states that the five major plant hormones, including auxin, are pleiotropic and indicates that each pleiotropic hormone has “more than one effect on the growth and development of plants”. The effects of auxin are then listed in detail to provide an example of the different effects a pleiotropic hormone can have on a plant. Thus, the specific effects of auxin are mentioned to illustrate the concept of pleiotropy as it is exhibited by plant hormones. Choice C can be eliminated because the specific effects that auxin has on plant development are not discussed in the context of the hierarchy of hormones. Choice A and B are incorrect because they cite topics that are not discussed in the passage.