Step 1: A delusion is a fixed, false belief that is firmly held despite clear evidence to the contrary and is not shared by others of the same culture or background.
Step 2: Here the woman holds an unshakable false belief that her husband is unfaithful and rejects all proof and reassurance from relatives. This jealous, evidence-resistant conviction is a delusion (specifically the delusion of infidelity, or Othello syndrome).
Step 3: The distractors are different: an illusion is a misperception of a real external stimulus, a hallucination is a perception without any external stimulus, and perversion refers to deviant behaviour, not a false belief.
Step 4: Because the core abnormality is a false fixed belief held against evidence, the answer is delusion.