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. Â 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.