Correct Option (C)
A black hole is characterized by an extremely high density. This immense density results in a gravitational field so powerful that nothing, not even electromagnetic radiation (like light), can escape its pull once it crosses the event horizon. The escape velocity at the event horizon exceeds the speed of light.
Incorrect Options:
Options A and B are incorrect because the size of a black hole, while a factor in its overall mass, is not the primary determinant of its inability to emit radiation. The critical factor is the concentration of mass within a very small volume, leading to extreme density. A black hole can be large or small in terms of its event horizon radius, but its defining characteristic is the extreme density at its singularity.
Option D is incorrect because a very low density would imply a weak gravitational field, allowing radiation to escape. The defining feature of a black hole is its exceptionally high density, which generates the powerful gravitational force responsible for trapping radiation.