Final answer:
The most appropriate form of the verb abrir to fill in the blank is abría.
Step-by-step explanation:
The most appropriate form of the verb abrir to fill in the blank in the sentence is OC. abría. The sentence is in the past tense and Ana was speaking on the phone at the same time Carla was opening the door, indicating a continuous action in the past.
Learn more about verb conjugation in Spanish