en las indicaciones de un sobre para preparar gelatina se dice que por cada litro de agua se debe poner un sobre completo ¿cuantas gelatinas de 20 mililitros se obtiene de un litro?

do you know the title of the listening so i can hear and answer this?

what list? there is not

corras is the only one that is informal but doesn't follow the suffix rule. correr is the verb. corre is the formal verb form. corra would be used as the formal is corre ended with an a.

your best bet would be corras, it's the only one that is informal but is still wrong