Answer is B. She is the person whom he had found sleeping in the library.
A. That author is the one who I saw at the bookstore today.
A would have been correct if it had the word 'whom' instead of who.
B. She is the person whom he had found sleeping in the library.
Its the correct option.
C. Against who did you think you would compete in the contest?
This word 'whom' is supposed to be used here in the place of who to make it correct.
D. The musical group whom won the top prize are from my hometown.
The use of the word 'whom' here makes the sentence incorrect.