Final answer:
The student's action of punching his little brother after being angry with his teacher is an example of the defense mechanism known as displacement. Displacement involves redirecting emotions to a less threatening target.
Step-by-step explanation:
The behavior described in the question is an example of the defense mechanism known as displacement. Displacement is a psychological defense mechanism in which a person transfers their feelings from the original source of stress onto a less threatening target.
Other defense mechanisms mentioned include reaction formation, where an individual behaves in a way that is opposite to their true feelings, and projection, where a person attributes their own unacceptable thoughts or feelings to someone else. However, in the given scenario, the correct answer to the student's question is D) Displacement.