Which aviation fuel is primarily used in piston-engine aircraft?

AVGAS, or aviation gasoline, is specifically formulated for use in piston-engine aircraft. It generally contains a higher octane rating than regular automotive gasoline, which is essential for the performance and efficiency of piston engines. AVGAS is designed to provide reliable ignition and combustion characteristics critical for aviation safety.

The other options do not apply to piston-engine aircraft in the same way. Diesel fuel is primarily used in diesel engine aircraft, which are a different class of engines not commonly found in traditional small aircraft. Gasoline-kerosene mixtures are not standard aviation fuels and would typically not be found in piston-powered planes. Jet A is intended for turbine-engine aircraft and is a kerosene-type fuel. Thus, AVGAS stands out as the appropriate choice for piston-engine aircraft, ensuring they operate safely and efficiently.