Solid-fuel, forced warm-air furnaces primarily burn which of the following?

Solid-fuel, forced warm-air furnaces are specifically designed to burn organic materials that are in solid form. This includes materials such as coal and wood. These furnaces operate by utilizing the heat generated from burning these solid fuels to warm air, which is then distributed through a duct system throughout the building.

The design and combustion process of solid-fuel furnaces require fuels that are in solid state, making coal and wood the primary choices. They can produce a significant amount of heat and are often used in scenarios where other fuel options, like natural gas or electricity, might not be available or cost-effective.

In contrast, other options mentioned involve different types of fuels: natural gas and propane are gaseous fuels; oil and kerosene are liquid; and electricity is an energy source rather than a fuel in the traditional sense of combustion. Therefore, these options do not apply to solid-fuel furnaces, confirming that coal and wood are the correct fuels for this type of heating system.