<prompt> Illusion is -Feeling of familiarity with unfamiliar thingAltered perception of real objectBelief in something even though the same is proved falsePerception occurring without external stimulation  <response>: Ans is b i.e. Alteration of perception of ones reality o Altered perception of real object - Illusiono False perception without external stimulus - Hallucination.Illusiono Illusions are altered perception in which a real external object is combined with imagery to produce false internal percept. In simple words, illusion is misinterpretation of an actual sensory input. For example:-i Hearing once name in a train whistle:- Train whistle is a real external stimulus, which is perceived as once name false internal percept.ii Mistaking a stick or rope for snake in dark room:- Stick is a real external object, which is perceived falsely as snake.In illusion:-i Sensory input is in external atmosphere.ii Percept is in internal atmosphere inner subjective space, i.e., inside the mind.o Illusions are most commonly seen in delirium. 