Step 1: Understanding the Concept:
In psychoanalytic theory, defense mechanisms are unconscious psychological strategies used to cope with anxiety and to protect the self-concept. The question asks to identify which option is not a defense mechanism.
Step 2: Detailed Explanation:
(A) Repression: An unconscious mechanism of pushing disturbing or threatening thoughts from awareness. It is a primary defense mechanism.
(B) Suppression: A conscious mechanism of pushing unwanted thoughts out of mind. While conscious, it is still considered a defense mechanism.
(C) Projection: A defense mechanism where an individual attributes their own unacceptable thoughts, feelings, and motives to another person.
(D) Ego: The Ego is one of the three components of personality in Freud's structural model (along with the Id and Superego). The Ego is the part of the personality that {uses} defense mechanisms to mediate between the demands of the Id, the Superego, and reality. It is not a defense mechanism itself.
Step 3: Final Answer:
Repression, Suppression, and Projection are all defense mechanisms. The Ego is the structure of the mind that employs them. Therefore, the Ego is not a defense mechanism.