Answer : Mientras Ana hablaba por teléfono, Carla abrió la puerta.
Explanation : In the main sentence appears the verb "hablar" conjugated in the past, and while this action occurred, there was the punctual act of opening the door, which ended at that moment, so the verb "abrir" must be conjugated in the past. Since the subject of that sentence is Carla, who is a third singular person, the verb "abrir" must be conjugated in the third singular person too. Ella abrió, tercera persona singular del pretérito del verbo abrir.
Hope this helps!
[tex]\textit{\textbf{Spymore}}[/tex]
