Answer:
The correct pronoun is "la".
Step-by-step explanation:
The correct pronoun is "la": El novio de Carla toca la guitarra. La toca en todas las fiestas. "La" is a pronoun which accompanies a singular female noun. In this case, that noun is guitarra (guitar). In Spanish guitarra is a female singular noun. "Lo" wouldn't be correct as it is singular but male. "Los" is male and plural. And "las" is female but plural also. The only correct option is "la".