Right answer
a. Las
So the dialogue is as follows:
Eduardo: Tuvimos que comprar muchas cosas
Víctor: ¿Las compraste o no?
This problem uses the concept of direct object pronoun that is a pronoun that replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. Since direct object pronouns in Spanish agree in number and gender with the nouns they replace, the direct object pronoun we must choose here stands for the third person plural given in feminine form because the noun cosas is feminine and plural, that is, las.
