Fui al mall para comprar un regalo para el cumpleaños de mi hijo y pagué el regalo con mi tarjeta de crédito.
Explanation:
Fui al mall para comprar un regalo para el cumpleaños de mi hijo y pagué el regalo con mi tarjeta de crédito. The verb is in first person and in the preterite: "I went to the mall to buy a present for my son's birthday and I payed the present with my credit card". The first option "pagar" is incorrect because it's the infinitive. The second, "pagaste" is incorrect because it is in the preterite but in the second person of the singular. The third, is also in the preterite but in the third person of the singular. Pagué is in the first person, meaning it was me that payed for the present.
