Right answer:
papel
Papel here translates into role and is an actor's part in a play or movie, etc. This is the more logical answer because actor means actor, escenario means scenario and cortina means curtain. On the other hand, this sentence stands for the simple present because creo is the conjugation of the verb creer that matches the first person singular in the simple present. Finally:
Creo que mi amigo juega bien su papel