Final answer:
The pelvis and femur in whales are vestigial structures, which have reduced in size over time because they no longer serve a function in whales and there is no selective advantage in maintaining them.
Step-by-step explanation:
In humans, the pelvis and femur are critical for bipedal locomotion. In contrast, whales have remnants of these bones, which do not serve a purpose for their aquatic lifestyle. These remnants are known as vestigial structures, which are structures that have no apparent function and are reduced in size compared to the functional structures in other organisms. Vestigial structures, like the pelvic bones in whales or the human appendix, are often reduced in size because they no longer provide a survival advantage, and there might be a selective pressure against the energy expenditure required to develop them.
Considering the facts presented, the correct answer to the student's question is B. vestigial structures. This is because the pelvis and femur in whales are leftover parts from their land-dwelling ancestors, which have shrunk over time due to lack of use. The presence of these structures supports the theory of evolution, indicating that whales share a common ancestor with land mammals that had fully developed pelvic and leg bones.