Explicit goes in the blank. :)
Answer:
B
Explanation:
Who and whose (which is wrong for a couple of reasons) refer to people. So neither of those 2 can be the answer.
Which and that are all that is left.
The usage is very close. Which, I think, is the correct answer. It usually begins a clause that adds more information to the noun (usually) that it modifies. In this case, the clause modifies mammal and tells it that is unique in that it can fly.
That is normally a pronoun that oddly modifies again usually a specific noun. You are not talking about a bat that lives in your back yard and that you have named. Bats in general are the only mammals that fly.
I still would use which.
