Juan Pablo va a comprar un auto a nosotros.
Juan Pablo va a comparnos un auto.
All other options are incorrect.

The indirect object replaces the one who receives the verb. It can be replaced, if it is in the third person, "le" (singular) or "les" (plural); they can also be replaced by unstressed pronouns (me, se, te, nos, os).
In this case, the indirect object is "a nostros" (to us), as it must fit with the tonic pronoun, which in this case is "nostros" (we) must be "nos".
In this case, since the verb is in the infinitive, the verb and the pronoun can be combined.
Note:
In the case that if the person who utters the sentence is different from the person who uttered the first sentence, it is possible that the person is not included in the word "nosotros" (we), in that case the correct option is option B . les.