Which of the following sentences is correct?           a. against who did you think you would compete in the contest?   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. the musical group whom won the top prize are from my hometown.  

I think that the answer is "B- That author is the one I saw at the bookstore today."
My reason is: When in doubt, try this simple trick: If you can replace the word with “he”' or “'she,” use who. If you can replace it with “him” or “her,” use whom. Who should be used to refer to the subject of a sentence. Whom should be used to refer to the object of a verb or preposition.

There is nothing popping up so what are u asking