Someone please help with this

When traveling in the spanish-speaking countries, it's very important to be able to understand directions in spanish, so that you can find out how to get from place to place. but what happens if you need to give a friend directions? would you be prepared? in this project you'll practice giving directions to places in panama city.

Fill the blank with a la, a las, a los, or al. carlos no en el parque.

El estudiante leía una novela de terror cuando, de repente, alguien _ a su puerta a. llamé b. llamó c. llamando d. llamaba