Final answer:
While refueling a placarded vehicle with the engine off, someone must always stay at the nozzle to prevent fires, spills, and other hazards.
Step-by-step explanation:
When refueling a placarded vehicle with the engine off, someone must always be in attendance at the nozzle.
This is a critical safety procedure to prevent any accidental fires, spills, or hazards associated with the hazardous materials being transported. The person attending must also be aware of the potential for static electricity discharge, which can ignite flammable vapors, and should thus ensure that all necessary precautions are taken to avoid such risks.