Final answer:
Under United States Department of Transportation regulations, a vehicle carrying hazardous materials must have 4 placards, one on each side and end.
Step-by-step explanation:
According to the United States Department of Transportation hazardous materials regulations, a vehicle carrying hazardous materials must display placards. These regulations require that placards be placed on each side and each end of the vehicle, which means a total of 4 placards are necessary. Each placard must clearly identify the specific hazard of the materials being transported to ensure safety and compliance with the regulations. This requirement helps in the event of an accident or spill, as responders can quickly identify the potential hazards and take appropriate action.