Right answer:
Nos interesa la biología
This answer is correct because it uses correctly two words. Since the question is being addressed to us, the response must match the first person plural. So nos is the indirect object pronoun that matches this person. Recall that an indirect object pronoun tells us to whom or for whom something is done. So here the action is being performed for us. On the other hand, since biología is a singular noun, interesa is the conjugation of the verb interesar that matches this singular form in the simple present.
