Relative pronouns are the pronouns that are used to introduce the
dependent clause in the sentence. It can be for either the object or the
subject.
Here, in this question, the relative pronoun with which we are
supposed to fill the blank should correctly represent the person "someone".
The relative pronoun used to refer to person are: "who", "that", "whose" & "whom".
"which" is used to refer to objects, thus it is inappropriate for use here, and this eliminates option C.
"whom" is used to refer to people when the person is the object of the verb.
Here in this sentence the person ("someone") is the subject of the verb. Thus it is inappropriate for use here, and this eliminates option D.
"that" can be used to refer both, either people or objects, whereas "who" is specifically used to refer to people.
Thus "who" is a more appropriate word for use here.
Therefore the complete correct sentence is:
There's someone at the door WHO wants to speak to George.