The response that would provide the strongest evidence for the most recent common ancestor among humans, whales, and horses is:
they all have similar bones in their limbs.
This is because similarities in limb bone structure are indicative of evolutionary relationships and common ancestry, reflecting adaptations to different environments while retaining an underlying structural blueprint. The presence of similar bones supports the idea of a shared ancestry, while the other options, although they indicate shared characteristics of mammals, are more general and less specific to a recent common ancestor.