Answer:
A. share one common ancestor that isn't also shared by organisms in other groups.
Step-by-step explanation:
A monophyletic group includes all the descendants of an ancestral species. On the other hand, a paraphyletic group includes some descendants of a common ancestor. Protists are not monophyletic as they share a common ancestor with other groups. Hence, protists are said to be paraphyletic. Mammals are monophyletic as all the mammals share a common ancestor which is not shared by organisms of any other groups.