Final answer:
Sexual dimorphism is the difference between the sexes of a species in body size or shape, due to different selective pressures from sexual selection, often resulting in males being larger or more ornamented than females.
Step-by-step explanation:
Sexual dimorphism refers to the differences in body size or shape between males and females of the same species that go beyond the reproductive organs. It is a consequence of sexual selection, where one sex typically males, may have more variance in reproductive success than the other, resulting in different selective pressures. This often leads to males being larger, stronger, or more ornamented than females, as seen in the elaborate tail feathers of a peacock compared to a peahen, or the size difference between male and female Argiope appease spiders. In some species, however, this role is reversed, and females may be selected for larger body sizes or more elaborate traits because they exhibit a greater variance in reproductive success.
The correct answer to the student's question is 'd) a difference between the sexes of a species in body size or shape', which encompasses the wide array of physical characteristics that can differ between males and females due to sexual dimorphism.