Final answer:
After failing a test, Sam's behavior of stomping out and slamming the door is indicative of the defense mechanism termed regression, reflecting a return to immature and child-like behavior.
Step-by-step explanation:
The defense mechanism Sam is likely demonstrating after failing the test is called regression. When people experience regression, they return to an earlier stage of development, displaying behaviors that are characteristic of a less mature stage of life. In the given scenario, by stomping out of the room and slamming the door, Sam is exhibiting behavior more typical of a young child having a tantrum, not an adolescent facing a disappointing outcome maturely.