The direct object pronoun to replace los libros is going to be the same definite article - los. This eliminates options A and D.
The indirect object in the new sentence will need to refer to las niñas. Let's look at the indirect objects in the remaining options.
B. Nos los leo.
- Nos --> refers to "us"
C. Me los leo.
- Me --> refers to self
E. Se los leo. - Se --> refers to "them"
Our indirect object is third-person plural. Se is used for a third-person plural indirect object.
Answer:
E. Se los leo.
