A direct object pronoun replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. Moreover, direct object pronouns in Spanish agree in number and gender with the nouns they replace. The Table below shows the different forms of direct objects in Spanish. So, we can answer these questions as follows:
a. ¿Ves a Juan y a Pablo?
The question stands for the third person plural, that is, Juan y Pablo is the direct object in this sentence. Therefore, we must replace the direct object of this sentence by the direct object pronoun that matches the third person plural, which is los. Thus, the response to this question is:
Los veo
Veo is the conjugation of the verb ver for the first person singular in the simple present.
b. ¿Compraste un regalo?
The direct object is un regalo, that stands for the third person singular. So we need to replace it by the direct object pronoun that matches the third person singular in masculine form, which is lo. Finally, this question can be answered as follows:
Lo compró
Compró is the conjugation of the verb comprar for the third person singular in the preterite tense.
c. ¿Me llamaste por telefono ayer?
The direct object matches the second person singular for you directly receives the action of the verb llamar in this sentence. So the direct object pronoun that matches the second person singular is te. Finally, this question can be answered as follows:
Te llamé
Llamé is the conjugation of the verb llamar for the second person singular in the preterite tense.
