Final answer:
Black holes are formed from supernova collapses of massive stars and possess gravitational fields so powerful that not even light can escape from them. They become invisible due to their intense gravity which pulls in matter and emits high-energy radiation like X-rays. The correct option is b. a collapsed star.
Step-by-step explanation:
A black hole is a cosmic body exhibiting gravitational forces so strong that nothing can escape from within it, not even light. The concept of a black hole emerged from the idea that if a body's escape velocity exceeds the speed of light, light would not be able to break free from the body's gravitational pull. This concept was revived after Einstein's theory of general relativity predicted such extreme gravitational effects.
Black holes are thought to form from the supernova collapse of a massive star. Such a catastrophic event compresses the core of the star into an incredibly small and dense point known as a singularity, surrounded by an event horizon. Beyond this point, the gravitational pull is so intense that the escape velocity exceeds the speed of light, making the black hole invisible, and distorting spacetime around it.
The evidence for black holes includes the observation of high-energy phenomena in space, like X-ray emissions from matter being pulled in and heated to extreme temperatures. Although black holes may not be directly observable, their influence on nearby matter and the detection of gravitational waves support their existence.
Given the multiple choices in the student's question, the correct answer is: b. a collapsed star.