Which type of fuel is classified as compression-ignited?

Compression-ignited fuel refers specifically to fuels that ignite by the heat generated from compressing the fuel-air mixture in the combustion chamber, rather than using a spark plug for ignition. Diesel fuel is the primary example of such fuel. In diesel engines, the air is compressed to such a degree that its temperature rises significantly, allowing the injected diesel fuel to ignite spontaneously when combined with the hot compressed air.

In contrast, gasoline and ethanol are classified as spark-ignited fuels because they require a spark from a spark plug to ignite the fuel-air mixture. Propane, while it can be used in internal combustion engines, is typically used in engines designed for spark ignition as well. Therefore, diesel's unique property of being ignited by compression makes it the correct answer in this context.