The response that provides evidence for the most recent common ancestor among humans, whales, and horses is:
they are all mammals.
This is because being part of the class Mammalia indicates a shared common ancestry that includes features such as mammary glands, hair (at some life stage), and other characteristics typical of mammals. The other options (hair, internal skeletons, similar bones in limbs) may also indicate common traits, but the classification as mammals is a more definitive indicator of a common ancestor that includes all three species.