Final answer:
Regression is the ego defense mechanism in which a person returns to a more immature behavioral stage. Sebastien's behavior of yelling at his wife after feeling resentful towards his boss is an example of regression.
Step-by-step explanation:
The ego defense mechanism in which Sebastien returns to a more immature behavioral stage is called Regression. In this case, after feeling resentful towards his boss, Sebastien yells at his wife upon arriving home. This behavior is an example of regression, where he is acting younger than his actual age and expressing his frustration in an immature way.