Final answer:
a. a point of infinite density. A singularity is a point of zero volume and infinite density that forms when an object collapses to become a black hole.
Step-by-step explanation:
A singularity, in the context of black holes, refers to a point of zero volume and infinite density. According to the theory of general relativity, any object that becomes a black hole must collapse to a singularity. It is the point at which gravity becomes infinitely strong, and anything that passes inside the Schwarzschild radius, the critical radius for a given mass, cannot escape.