Final answer:
The whale is most closely related to the hippopotamus, as both are descended from even-toed ungulates and share a common ancestry, based on molecular data and phylogenetic trees.
Step-by-step explanation:
The closest relative to a whale, based on molecular data and phylogenetic trees, is not a shark, dolphin, or cow, but rather option C, a hippopotamus. This is because Cetaceans, the group that includes toothed whales such as dolphins and killer whales, as well as baleen whales like the humpback whales, are descended from even-toed ungulates (hoofed animals with an even number of digits) and share a common ancestry specifically with the hippopotamus, among other ungulates.