What is a black hole?

A black hole is defined as an area of space with a gravitational pull so strong that nothing can escape from it, including light. This occurs when a massive star collapses under its own gravity at the end of its life cycle, concentrating a significant amount of mass within a very small volume. The escape velocity in this region exceeds the speed of light, which is why it is termed a "black hole," as no light can escape, rendering it invisible to our eyes and telescopes.

In the context of the other options, a large star that has exploded refers to a supernova, which may create a black hole if the remnant core is sufficiently massive. However, a black hole is not simply an exploded star; it is the end state after such an explosion if the conditions are right. A dark region of a galaxy may refer to various phenomena, but it does not accurately define what a black hole is. Furthermore, describing a black hole as a planet with extreme gravity is misleading; planets are not typically associated with such extreme gravitational field strengths nor are they formed in the same manner as black holes. Thus, the definition that accurately describes a black hole is the one that highlights its unique gravitational properties, as captured in the correct answer.