In which type of heating system does water pass through a heated coil?

The correct choice involves a system where water is heated and then circulated through a coil, which is typical in an instantaneous water heater. In this type of system, water is heated on demand as it flows through a heated coil. This is efficient because the water does not need to be stored, reducing the risk of heat loss and providing hot water immediately when needed.

The other options do not fit this description as accurately. A boiler system typically heats water in a tank and distributes it through radiators or underfloor heating, rather than having it flow continuously through a heated coil. An electric radiator uses electric elements to produce heat directly rather than circulating heated water. Heat pumps transfer heat rather than heating water directly through coils. Each of these systems operates on different principles of heating and water circulation.