Which of the following sentences is correct?

A. That author is the one who I saw at the bookstore today.

B. Against who did you think you would compete in the contest?

C. She is the person whom he had found sleeping in the library.

D. The musical group whom won the top prize are from my hometown.

It is c that is the right answer

It might be said that the correct sentences is C "She is the person whom he had found sleeping in the library". This is because whom is correctly used.

In the case of A and B, They are incorrect because instead of who it should have been whom, the correct pronoun.

On the contrary; sentence D should be who instead of whom.