This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Psychiatry
Persistent belief in something which is not a fact is - 
Select and explain. 
A. Illusion, B. Hallucination, C. Delusion, D. Delirium
C: Delusion
Answer: C: Delusion 
Explanation: A delusion is false, firm (unshakable) belief which are not in keeping with patient's socio-cultural and educational background.