C. nos
An indirect object pronoun is used to tell us to whom or for whom something is done. In this case, I went to school to our first class of the year, so the profesor talked about the rules of the University. Since the action is being performed for us, that is, he talked to a group of students, then we choose the indirect object pronoun that matches the first person plural, which is nos. Therefore:
Ayer fui a nuestra primera clase del año y el profesor nos habló de las reglas de la universidad
