Gusta, since the singular form of gustar (gusta) is used for singular nouns (such as libro, helado, casa, etc.) and infinitive verbs (such as mirar, leer, comer, etc). Gustan is used for plural nouns (such as libros, helados, casas, etc). Hope this helps!
I had a question like this for my class a long long time ago when I was in elementary school kind of and if I’m correct the answer was B for my class but it could be different for you
