Step 1: Know what a tranquilizer is. Tranquilizers are medicines used to reduce stress and to treat mild or serious mental illness. They calm the nervous system.
Step 2: Recall common examples. Well-known tranquilizers include compounds like barbiturate and sulpha or amine type structures, such as Veronal.
Step 3: Understand the question. The question shows four structures and asks which one is a tranquilizer. We must pick the structure that matches a known tranquilizer.
Step 4: Compare the structures. Three of the structures match other drug classes. The fourth structure has the sulfonamide-amine pattern that fits a medicinal tranquilizer.
Step 5: Identify the match. Based on standard drug structures, structure 4 matches the tranquilizer profile.
Step 6: State the answer. So the structure that represents a tranquilizer is structure 4. \[ \boxed{\text{Structure 4}} \]
