Question

what are these structures an example of?
transitional structures
analogous structures
vestigial structures
adapted structures

Explanation:

Vestigial structures are remnants of organs or structures that had a function in an early ancestor. In whales, the pelvis and femur are vestigial. Whales evolved from land - dwelling ancestors. Over time, as whales adapted to an aquatic lifestyle, these structures (pelvis and femur) lost their original function (like supporting legs for walking on land) but remain as evolutionary leftovers. Transitional structures would show a "middle" form in evolution (not the case here). Analogous structures have similar functions but different evolutionary origins (not relevant here). Adapted structures imply current functionality which these (pelvis and femur in whales) lack in their current form.

Answer:

vestigial structures