Answer: Sí, les gustan las fiestas. OR No, no le gustan las fiestas.
Explanation:
The question is asking whether your friends like parties. Sí, les gustan las fiestas, is saying that yes my friends like parties. No, no le gustan las fiestas, is saying no, my friends don't like parties. The verb Gustar means to like. Everything must agree, so singular words like a dog would get "gusta" (Me gusta un perro.) wheras plural words like dogs, get "gustan" (Me gustan los perros) Hope this makes sense!
