Final answer:
The scenario described is an example of sublimation, where an unconscious desire to control others is transformed into a socially accepted role as a prison guard.
Step-by-step explanation:
The scenario presented with a middle-aged man who has an unconscious desire to control others and becomes a prison guard to fulfill this need is an example of sublimation. Sublimation is a defense mechanism where socially unacceptable impulses or idealizations are unconsciously transformed into socially acceptable actions or behavior. In this case, the man's desire to control others is redirected in a socially functional role.