Correct answer:
doy
Doy is the correct conjugation of the verb dar for the first person singular in the simple present. So the subject pronoun that matches this person in Spanish is yo as indicated in the sentence. On the other hand, les is the indirect object pronoun for the third person singular. An Indirect Object Pronoun is a person that receives the action of a verb indirectly. Its purpose is to tell you to whom or for whom something is done. In this sentence, the action is being performed for them, that's why we choose les. Finally:
Yo les doy el libro a mis hermanos.