Answer:
a. Astronomers have never seen a black hole.
Step-by-step explanation:
A black hole is 1-Dimensional point in space-time having density is extremely large such that space-time curvature bends enough for even light cannot escape. All the mass is concentrated at this point. A black hole is formed when a very high mass star dies. It goes supernova and collapses to form a black hole. It sucks in matter like a vacuum cleaner.
Astronomers have seen a black hole by actually viewing the event horizon which is a bright disk surrounding a black hole. Recently an image of a black hole was also released.