Question

which observation best shows that whales and hippos share a common ancestor?
both have tails on their bodies. a whale has a horizontal fluke, while a hippo has a short tail.
both have adaptations for water. a whale has a blowhole, while a hippo has high nostrils.
both have similar limb bones. a whales limbs form flippers, while a hippos limbs support walking.
both have streamlined bodies. a whales body is fully streamlined, while hippos are semi - streamlined

Explanation:

Similar limb - bone structures are strong evidence of a common ancestor as they are homologous structures. Homologous structures indicate that different species have evolved from a shared ancestor and have modified the same basic anatomical structures for different functions. Tails, water - adaptations, and body shapes can be due to convergent evolution or other factors and are not as strong evidence as homologous limb bones.

Answer:

Both have similar limb bones. A whale's limbs form flippers, while a hippo's limbs support walking.