Eduardo: ¿Compraste el libro para Mateo?
Fernando: Sí, _______ compré.
Translation:
Eduardo: Did you buy the book for Mateo?
Fernando: Yes, [blank] I bought.
The options
A. los - the ~masc. plural (definite article)
B. les - them, you ~masc. plural (indirect object pronoun)
C. las - the, them ~fem. plural (definite article, indirect object pronoun)
D. lo - the, it, him ~masc, singular (definite article, direct object pronoun)
First of all, our direct object (el libro) is masculine singular. That eliminates all options but one. Another thing that points to (D) as the correct answer is that none of the other options are direct object pronouns.
D. lo