A star that collapses into a singularity with zero volume and infinite density becomes a black hole. This is one of the most mysterious and fascinating phenomena in space. A black hole forms when a massive star ends its life and its core collapses under the force of gravity. If the mass is large enough, nothing can stop this collapse. Eventually, the star shrinks to a point where it has no volume, but all its mass remains, leading to what scientists call a “singularity.”

At this singularity, the density becomes infinite and the gravitational pull is so strong that not even light can escape it — that’s why it’s called a black hole. The boundary around this singularity, from where nothing can escape, is known as the **event horizon**.

Let’s understand why the other options are incorrect:
- **White dwarf**: This is a small, dense star that forms when a medium-sized star runs out of fuel. It does not collapse into a singularity.
- **Blue giant**: These are massive, hot stars that are still burning fuel and are in an earlier stage of life.
- **Wolf-Raynet star**: These are very hot and massive stars, often nearing the end of their lives, but they haven’t collapsed yet.

Black holes can grow by absorbing mass from nearby stars and dust. They are found at the centers of galaxies and in space where stars once lived. Studying black holes helps us understand gravity, time, and space better.

So, the correct answer is **Black hole** — the final, extreme stage of a massive star’s life.