The correct answer is 'nos divertimos' because the subject 'Mis amigos' is plural, indicating that the action of having fun is shared among them. In Spanish, the reflexive pronoun must agree with the subject in number and person. 'Nos' is the first-person plural reflexive pronoun, which means 'we' or 'us', and is appropriate here as it indicates that the friends had fun together. The other options either use the wrong reflexive pronoun or conjugation for the subject.