Final answer:
The placement of whales within the artiodactyls is supported by the morphology of their limb bones in extinct species and DNA evidence, making option 4 the correct answer.
Step-by-step explanation:
The placement of whales within the artiodactyls is supported by both the morphology of limb bones in extinct whales and DNA evidence. The correct answer to the question is option 4: a and b are correct. Firstly, the morphology of limb bones in extinct whales such as Pakicetus and Rodhocetus provides evidence that present-day whales evolved from terrestrial ancestors that had limbs equipped for walking.
Furthermore, DNA evidence corroborates this relationship by showing that whales share a common ancestry with modern day artiodactyls, such as the hippopotamus. When considered together, these two lines of evidence - morphological and genetic - provide a compelling argument for the evolutionary placement of whales within the artiodactyl group. While some artiodactyls do spend significant time in water, this is not direct evidence of the specific evolutionary relationship between whales and artiodactyls, thus making option 3 an incorrect reason for their placement within the group.