Which of the following sentences is correct?
a. the musical group whom won the top prize are from my hometown.
b. that author is the one who i saw at the bookstore today.
c. she is the person whom he had found sleeping in the library.
d. against who did you think you would compete in the contest?

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