What is a black hole?

A black hole is defined as a region of space where the gravitational pull is so strong that nothing can escape from it, not even light. This extreme gravitational force arises from the collapse of massive stars after they have expended their nuclear fuel. As the star undergoes this collapse, it compresses its mass into an incredibly small and dense area, leading to the creation of the black hole.

The defining characteristic of a black hole is its event horizon, which is the boundary beyond which nothing can escape the gravitational pull. Once an object crosses this threshold, it cannot return, making it effectively invisible to external observers. This unique property is what distinguishes black holes from other astronomical objects.

Other options may describe aspects related to black holes but do not fully capture their essence. For instance, while a black hole is indeed related to a star at its final lifecycle stage, it is more accurately defined by the characteristics of its gravitational pull and the region it creates in space. Additionally, while high density and mass are features of black holes, they do not specifically convey the concept of the gravitational point of no return. Therefore, the correct choice encapsulates the fundamental nature of a black hole perfectly.