C is the answer you are looking for. A common idea isn't a moral, the generalization is the Main Idea, but not quite the moral. The thought of the characters almost never have the moral in them. Therefore, C is your answer.
I believe that the best answer to this question would be a tragedy. However, there may be more than one genre that accomplishes this goal. A tragedy is, specifically, a form of drama. It, "treats in a serious and dignified style the sorrowful or terrible events encountered or caused by a heroic individual" (this is the definition given in Encyclopedia Britannica.) A tragedy most definitely depicts the flaws in human nature.
1. The Pope thinks Faustus is a ghost.
Faustus and Mephastophilis have become invisible and they came to the Pope to play tricks on him. They started flinging the dishes around the room and the Pope and his friars got scared and though there is a ghost in the room, which is why he started crossing himself.
2. Faustus tells the Pope to stop crossing himself because the sign of the cross has no effect on Faustus.
Faustus realizes that religion is a fraud, especially when he didn't see God or knows of his existence, whereas the Devil is next to him playing tricks on mortals and taking their souls. He isn't frightened or offended by those symbols - he just knows that they are future and have no effect whatsoever.
