Final answer:
When a black hole is rotating, it forms a ring singularity rather than a point singularity, due to the rotational kinetic energy and the effects of relativistic physics.
Step-by-step explanation:
When a black hole is rotating, the singularity at its core changes in shape compared to a non-rotating black hole. Rather than being a point of infinite density, it becomes a ring singularity. This is a consequence of the rotational kinetic energy and the laws of relativistic physics, which suggest that the spacetime around a rotating black hole, also known as a Kerr black hole, is different from that around a non-rotating black hole (Schwarzschild black hole). In the case of a rotating black hole, the central singularity forms a ring-like structure due to the centripetal forces resulting from the rotation. Hence, the correct answer to what changes about a black hole singularity when the black hole is rotating is c) It forms a ring singularity.