Final answer:
The correct form of the verb 'abrir' in Spanish is 'abran'.
Step-by-step explanation:
The correct form of the verb abrir that correctly completes the professor's statement is abran.
Profesora: Todos los libros, por favor.
The verb abrir is conjugated in the imperative form to address a group of people, hence abran is the correct form.
Learn more about Spanish verbs