False unshakable belief is termed as:

Correct option is C

A delusion is defined as a false, fixed, and irrational belief that is resistant to reasoning or confrontation with actual fact. It is a key feature of psychotic disorders, such as schizophrenia. Despite evidence to the contrary, a person with delusions continues to believe them as truth.
Explanation of options: (a) Illusion – This is a misinterpretation of a real external stimulus, such as mistaking a rope for a snake. It’s related to sensory errors, not beliefs. (b) Hallucination – This is the perception of a stimulus (like hearing voices) without any external cause. Unlike delusion, it's a sensory phenomenon. (c) Delusion – ✔️ This is the correct answer. Delusions are deeply held false beliefs that persist despite contradictory evidence. (d) Hypochondriasis – This is a condition where a person is preoccupied with having a serious illness despite medical reassurance. It’s not a false belief in the same fixed psychotic sense.