Correct answer:
C. Si, les gustan las fiestas.
Les is the indirect object pronoun for the third person plural. An Indirect Object Pronoun is a person that receives the action of a verb indirectly. Its purpose is to tell you to whom or for whom something is done. So the action in this sentence is being performed for my friends (for them). Since mis amigos is a plural noun, we choose gustan as the conjugation of the verb gustar. Finally:
¿A tus amigos les gustan las fiestas?
Sí, les gustan las fiestas
