What defines a region that has a gravitational pull so strong that nothing can escape from it?

A region that has a gravitational pull so strong that nothing can escape from it is defined as a black hole. Black holes are formed when a massive star collapses under its own gravity at the end of its life cycle. The gravitational pull of a black hole is so intense that it overwhelms all other forces, drawing everything, including light, into it. This is why they appear "black," as no light can escape their grasp, making them invisible against the backdrop of space.

In contrast, the other choices represent different astronomical phenomena but do not fit the description provided in the question. A white hole is theorized as the opposite of a black hole, from which matter and light can escape but cannot enter. A neutron star is an incredibly dense remnant of a supernova, but it does not have the escape-velocity limitations of a black hole. The event horizon refers to the boundary surrounding a black hole beyond which nothing can escape, but it is not itself the region that causes the escape issue; it is simply the threshold. Thus, the correct identification of the gravitational region described is indeed a black hole.