The tiny, non-functional hind leg bones (and pelvic bones) found in the bodies of modern whales are considered vestigial structures. These are remnants of anatomical features that were functional in an organism's ancestors but have lost their original purpose over time due to evolution and adaptation to a different environment (in this case, a fully aquatic one). Their presence provides strong evidence that whales descended from four-legged, land-dwelling mammals that walked on land around 50 million years ago.

A. are not related to land animals. This is incorrect because the presence of these structures is key evidence of a shared evolutionary history and common ancestry with terrestrial mammals, such as even-toed ungulates like hippos, cows, and deer.

C. have never had legs in their evolutionary history. This is incorrect because fossil records show a clear transitional series of whale ancestors with progressively smaller, but functional, hind limbs.

D. bones serve a major function in swimming. This is incorrect due to the fact that, the bones are tiny, internal, and not connected to the rest of the skeleton or used for locomotion. Whales use their powerful tail flukes for swimming, not these vestigial bones. (Note: some recent research suggests they may play a minor, secondary role as an anchor for reproductive organs in some male whales, but their major ancestral function was for walking, and they are non-functional in the context of movement).