Final answer:
All options presented, A) Heat pump condensing hot water system, B) Oil conventional tank hot water system, C) Induced draft gas-fired hot water system, and D) Tankless propane hot water system, are valid domestic hot water systems.
Step-by-step explanation:
Regarding which option is not a valid domestic hot water system, the choices given are all typically valid and currently in use. A heat pump condensing hot water system operates by transferring heat from a cooler to a warmer environment, utilizing energy more efficiently than conventional heating methods. Oil conventional tank hot water systems and induced draft gas-fired hot water systems are also common methods for heating domestic water, with the former using oil and the latter using natural gas as fuel sources. Tankless propane hot water systems provide hot water on demand without the need for a storage tank and are likewise a valid hot water heating system.
Thus, since all options presented (A through D) are actual, viable hot water systems, the question appears to contain a mistake as there is no invalid system among the choices provided.