What type of energy conversion occurs in the APU to drive compressor and accessories?

The correct choice is focused on the conversion of heat energy to horsepower, which accurately describes the primary function of the Auxiliary Power Unit (APU) in an aircraft. The APU operates by burning fuel to produce heat energy, which is essential for the process of generating mechanical energy.

This mechanical energy drives the compressor and various accessories necessary for the aircraft's systems. The APU essentially transforms thermal energy from combustion into useful work, measured in horsepower, which is critical for starting engines and providing power to pneumatic and hydraulic systems on the aircraft.

In contrast, the other options do not align with the fundamental operation of the APU. While mechanical to electrical energy may apply in other systems, the APU specifically converts heat from fuel combustion. Electric to thermal energy pertains more to electric heating elements rather than the work performed by an APU. Pneumatic to hydraulic energy conversion is not the primary focus of an APU's operation, as it does not function directly to transform those forms of energy.