Final answer:
The most critical safety hazard to report with a gas water heater is a TPR valve without a proper pipe to the floor, which poses immediate risks of scalding and damage in case the valve discharges.
Step-by-step explanation:
The condition with a gas water heater that should be reported as a safety hazard is A. A TPR (Temperature Pressure Relief) valve without a proper pipe to the floor. The TPR valve is a critical safety device that releases pressure if the water heater's temperature or pressure becomes too high. Without a discharge pipe directing the flow to a safe location, usually close to the floor to minimize the risk of scalding and damage, the hot water and steam from a discharging TPR valve can cause serious injury and property damage. It is essential to ensure that the TPR valve has a proper discharge pipe to mitigate any risks associated with a potential over-pressure event.
Other aspects like a leaking tank (B), rusting in the heating chamber (C), and missing union fittings (D) also represent potential problems, but the absent discharge pipe from the TPR valve poses an immediate safety risk and should be addressed promptly to ensure safe operation of the gas water heater.