Final answer:
Hair in mammals is a shared derived character, while having four limbs is D. a shared ancestral character unique to mammals.
Step-by-step explanation:
Hair in mammals is a shared derived character, which means it is a characteristic that evolved within the mammal clade.
This trait is unique to mammals and helps distinguish them from other animals.
On the other hand, having four limbs is a shared ancestral character, which means it is a trait that was inherited from a common ancestor and is also found in other groups of animals, such as reptiles, amphibians, and birds.
So, the correct answer is D. a shared ancestral character; a shared derived character unique to mammals.