Which of the following is not one of the common defense mechanisms?

Introspection is the correct answer because it does not qualify as a defense mechanism in psychological terms. Instead, introspection refers to the process of examining one's own thoughts, feelings, and motives. It involves self-reflection and gaining insight into one’s internal experiences.

On the other hand, repression, reaction formation, and displacement are all recognized defense mechanisms that individuals use to cope with anxiety or stress. Repression involves unconsciously blocking unpleasant thoughts or memories from awareness. Reaction formation is when someone behaves in a way that is opposite to their true feelings as a way of managing anxiety. Displacement occurs when a person redirects their feelings from the original source of distress to a more acceptable or less threatening target. Thus, introspection stands apart from these mechanisms as it does not serve a protective function in the same way that the others do.