B is correct. To talk about people, we say who, to say about objects we use which. So obviously, it cannot be C or D, because she is a person, not an object ;) Whose is a word that asks a question "Who does it belong to?", i.e. whose is this bag? - Who owns this bag? That's why A isnt't correct. Who's is correct, since it's "who is" and hence "Ask the girl WHO IS (who's) sitting over there. See the difference? Can you pass me the bottle which is in the corner? Can you ask this guy who's learning Japanese? Whose is this blue car?

It's B. "Whose" does not mean "who is". Since you are saying "Ask the girl who is sitting over there", you should use "who's" which is a contraction (abbreviated form) for "who is". People aren't referred to using "which", so C and D are out.