Answer:
le
Step-by-step explanation:
Ella le da libros a los chicos is the correct way. It translates directly into She gives the books to the kids. Since we are speaking of a single third person carrying and action, we must use "le". Using "les" would imply third person plural, so if it was a group of people giving the books and instead of "ella" the pronoun was "ellas" or "ellos", we should use "les" and it means it's an action being carried out by a group.