The correct answer is,whom
A backboard could mean:
a board placed at or forming the back of something, such as a collage or piece of electronic equipment.<span>
an upright board behind the basket, off which the ball may rebound.
a board used to support or straighten a person's back, especially after an accident.
It depends on the context of the word.
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It would be letter a, since an interrogative sentence is question. Since the first answer is the only one with a question mark, the answer is a.
