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.
In laws complete the relationship
You use está because talking to teachers, older people, grandparents, or crowds should be formal
No it’s not is just a name