Final answer:
The pair with the most shared derived characteristics is Cow, Horse, as they are both even-toed ungulates and have a more recent common ancestor than the other pairs listed.
Step-by-step explanation:
The pair that has the most shared derived characteristics would be C. Cow, horse. This is because they are both part of the same order, Artiodactyla, or even-toed ungulates, and share a more recent common ancestor compared to the other pairs listed. Cetaceans, which include dolphins, are indeed descended from even-toed ungulates, but they have diverged significantly and adapted to life in the water, resulting in them sharing fewer derived characteristics with horses than cows do. On the other hand, spiders are arachnids and cows are mammals, placing them in entirely different classes of the animal kingdom with a very distant common ancestry. Hence, the pair of cow and horse have the most in common in terms of their evolutionary history and genetic similarities.