Final answer:
The hardest feature of mammals to trace through the fossil record is hair because it does not preserve well, unlike other features such as mammary glands or viviparity.
Step-by-step explanation:
The evolution of which of the following features of mammals is hardest to trace through the fossil record? The most difficult feature to trace is hair. While characteristics such as mammary glands, placenta, and viviparity can be inferred from fossilized bones or impressions, hair does not fossilize well and leaves little to no direct evidence in the fossil record. Characteristics like mammary glands may leave marks on bones where muscles were attached, and viviparity can be sometimes inferred from fossilized embryos found inside the mother. However, hair is rarely if ever, preserved in fossils, making it exceedingly difficult to trace its evolutionary development.
The evolution of the feature hardest to trace through the fossil record among mammals is d) Viviparity. Viviparity is the condition in which the young develop within the female and are born alive, receiving nourishment from the mother's blood through a placenta. Since the fossil record mainly consists of fossilized bones and teeth, it is difficult to directly trace the development of viviparity through fossils, as placenta and soft tissues do not fossilize easily.