Can someone answer this Spanish question?

Ser/ir: past tense in this group. both verbs have the conjugation.

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

Asalt water lake in the mountains in the mountains a) nevado tres cruces in copiapó, chile, where you can hike and explore through nature near the highest active volcano in the world, volcán ojos del salado. b) laguna verde a destination for the adventure and nature lovers where you can explore the desert region seeing rare desert flowers and even do some sand surfing c) atacama an important archeological site for the chimú civilization near the city of trujillo, peru, where you can explore the sun and dragon pyramids as well as other ancient temples and palaces d) chan chan an important national park near copiapó, chile where you can climb through the andes mountains and enjoy the view of flamingos, flowers and much more