Final answer:
Denial is the unhealthy defense mechanism among the options given, where a person refuses to acknowledge the reality of a painful or difficult situation.
Step-by-step explanation:
The student asked which of the following is NOT a healthy defense mechanism: A) Sublimation, B) Repression, C) Rationalization, D) Denial. The healthy defense mechanisms include sublimation, where socially unacceptable impulses are transformed into socially acceptable actions or behavior, while the unhealthy ones generally involve a denial or distortion of reality. Denial is an example of an unhealthy defense mechanism, where the reality of a situation is refused to be acknowledged, and a person acts as if a painful event, thought, or feeling does not exist despite evidence to the contrary.