The correct answer is WHO.
Who is used to talk about the subject, the one who does the action in the sentence, the doer.
Ex: That's the girl who dates my best friend.
- Who dates my best friend? That girl dates my best friend. (Girl here is the subject)
Whom is used to talk about the object, the one who receives the action done by the subject.
Ex: That's the girl with whom I date.
- Who do you date? I date that girl. (Girl here is the object)