14. (who, whom) are you going to the park with?

Whom

Explanation:

The quick test in choosing between who and whom is to substitute he or him. If he sounds better, who is correct; if him sounds right, whom is correct. That’s because as a pronoun whom is used to represent the object of either a verb or a preposition, while who represents the subject of a verb.

He is the consultant whom we contacted for advice. (We contacted him.)

To whom was the letter addressed? (The letter was addressed to him.)

He is the consultant who can answer your question. (He can answer your question.)

Who did you go to the movies with? is technically incorrect but very common, even for speakers who are well aware of the mistake.

With whom did you go to the movies? is correct but in an informal conversation can stand out as having a bit more formal tone.

Hope this helps :)