The answer is D: He is whom you want to speak to.
Whom is a pronoun used when referring to the object of a verb or preposition. Who, on the other hand, is used when referring to the subject of a sentence. The rule of thumb to remember is that, if you can replace whom with him or her, then you should use whom; and, if you can replace who with he or her, then you should use who. In the answer above, if the sentence is restated in a simpler way, you could say: you want to speak to him.
