Answer: lo
Eduardo: ¿Compraste el libro para Mateo? Fernando: Sí, lo compré.
Translation:
Eduardo: Did you buy the book for Mateo? Fernando: Yes, I bought it.
Explanation: "Comprar" is a transitive verb and needs a direct object to complete its meaning. If we want to replace a direct object with its pronoun, it has to agree in gender and number with the direct object.
Since the direct object in this sentence is "el libro", which is a masculine and singular noun, the direct object pronoun is "lo". Lo compré.