In B, "walk" is not a preposition, It's a verb >>we reject the answer. In C, "wither" is not a preposition. >>we reject the answer. In D, "whale" is not a preposition, it's a noun. >>we reject the answer.
so A is the correct asnwer!
ps. it there are also other problems in the rejected answers, for example, "wonder" is a verb, but one false word is enough, so I just gave an example.
