Which of the following is a type of aircraft engine?

A reciprocating engine is indeed a type of aircraft engine that operates using pistons to convert the pressure of combustion into rotational motion. This type of engine is commonly found in smaller aircraft and works on the principle similar to that of a car engine, where fuel is compressed inside a cylinder and ignited, the resulting explosion pushes a piston that turns a crankshaft. The use of reciprocating engines is central in aviation for various types of light aircraft and also in some military aircraft.

The other options, while associated with aircraft systems, do not classify as types of aircraft engines. Hydraulic systems are used for controlling various aircraft operations, such as landing gear and flight control surfaces, but they do not generate propulsion. Pneumatic systems involve the use of compressed air for various functions but are not engines themselves. Electric engines are indeed becoming more prevalent in aviation, particularly in hybrid systems or electric aircraft, but the term traditionally refers to electric motors which are not classified in the same way as reciprocating and turbine engines within aviation history and classification.