**Core Concept:**
The question refers to a psychological phenomenon where a person experiences an illusion about a familiar object, person, or situation. In this case, the person perceives a close friend has been replaced by an exact double, which is a type of confabulation. Confabulation refers to the production of false memories, beliefs, or explanations that can be shared with others or believed in by oneself.

**Why the Correct Answer is Right:**
The correct answer is Option D, "confabulation." Confabulation is a type of illusion in which the individual creates false memories, beliefs, or explanations that can be shared with others or believed in by oneself. In this scenario, the person experiences a false memory of a close friend being replaced by an exact double, which is an example of confabulation.

**Why Each Wrong Option is Incorrect:**
A. **Option A (hallucination):** Hallucinations are sensory experiences without an external stimulus, typically involving the visual or auditory system. In contrast, confabulation involves the creation of false memories and explanations, as in the provided scenario.

B. **Option B (delusion):** Delusions are false beliefs that are resistant to logical argument or evidence and are not shared by others. While the person in this scenario holds a false belief, the explanation is focused on the creation of false memories rather than a belief that cannot be changed or is not shared with others.

C. **Option C (hallucination):** See explanation above for Option A, as hallucination is not the correct answer for this scenario involving false memories and explanations.

**Clinical Pearl / High-Yield Fact:**
Confabulation can be observed in neurological disorders like stroke, dementia, or traumatic brain injury. This phenomenon helps differentiate between normal human cognition and neurological disorders, as confabulation is not typically seen in healthy individuals.

**Correct Answer:** D. Confabulation