Ablack hole is
a - an empty region of space with a huge gravitational field
b - a small region that contains the mass of may galaxies
c - simply the remains of a giant star that has undergone gravitational collapse

A black hole is simply the remains of a giant star that has undergone gravitational collapse. As the statement mentioned in option A like empty region, that cannot be considered as correct one. Because the black holes, the region of space with high gravitational pull and formed after massive star collapse. So, correct answer would be C.

Option C

Explanation:

A black hole is a dense region in the space where the gravitational pull is maximum. Due to large amount of gravitational pull in that region, no matter even light cannot pass through the black hole. It is surrounded by very high energy and it even has the capacity to attract materials towards itself.

If a material enters the black hole, it will be completely destroyed. It is stated that black holes are formed by the gravitational collapse of giant stars after their decay.

So once the giant star decay they will collapse and their gravitational force will tend to condense the remains of the giant stars and they will be circulating with high gravitational pull forming the black holes.