What Do We Mean by the Singularity of a Black

What do we mean by the singularity of a black hole?

A) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole.
B) The term is intended to emphasize the fact that an object can become a black hole only once, and a black hole cannot evolve into anything else.
C) It is the edge of the black hole, where one could leave the observable universe.
D) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions.