What is the prisoner's fate? A
Because he discovered the pit, the Inquisitors decide to make him want to be thrown in through threat of torture,

B
Since he didn't fall into the pit, the Inquisitors decide to kill him with a blade that he can see approaching.

C
The Inquisitors are worried the narrator will jump in the pit and spoil their fun, so they're killing him by guillotine.

D
The Inquisitors are letting the narrator choose between death by pit, pendulum, or hungry rats.