The correct answer is letter C. She is the person whom he had found sleeping in the library.
A. The musical group whom won the top prize are from my hometown (WRONG) Because of the word 'whom' which supposedly 'who'
B. That author is the one who I saw at the bookstore today. (WRONG) Whom instead of who
D. Against who did you think you would compete in the contest? (WRONG) It should be whom instead of who
