The correct choice is D: Faustus does not truly understand Christianity.
In the fragment, Faustus asks about a paradox of Christianity. On the one hand, if we say that we are clean of sin, we lie. The lie is a sin. On the other hand, if we sin we will inevitably die. There is no escape to our death.
What Faust does not understand is his own limitation about life. The eternal life is not a possibility for the humanity. As men and woman, we must die. Only gods are eternal and wanting to be like gods is the greatest sin existing.
