What type of heating system uses gas as a fuel source?

The gas furnace is the correct answer because it specifically uses natural gas or propane as its fuel source for heating. Gas furnaces operate by igniting the fuel, which then heats air that is circulated throughout a building via ductwork. This type of heating system is widely used due to the efficiency and rapid heating capabilities of gas as a fuel.

In contrast, the electric heat pump relies on electricity to transfer heat rather than generating heat from fuel combustion. Geothermal heating systems utilize the earth's natural heat and also do not rely on gas. Hydronic heating systems circulate hot water through pipes, which can be heated by various sources, including gas, but are not exclusively gas-based. Therefore, the gas furnace stands out as the type of heating system that primarily and directly uses gas as its fuel source.