Which characteristic is indicative of a reciprocating engine?

A reciprocating engine is characterized by its use of pistons that move back-and-forth within cylinders to convert fuel into mechanical energy. This movement is a fundamental aspect of how the engine operates, as it involves the cyclical intake, compression, power, and exhaust of gases, all facilitated by the pistons' reciprocating motion.

The reciprocating motion allows for a very efficient conversion of linear motion into rotary motion, but the key characteristic that distinguishes a reciprocating engine is the piston's movement. This back-and-forth motion drives the crankshaft, ultimately enabling the engine to perform work effectively.

In contrast, the other options do not accurately describe the essential nature of a reciprocating engine. While it does create rotary motion, it is not primarily about efficiency in that regard. Combustion is essential for the operation of this type of engine, and it does operate in cycles, not continuously, as each cycle corresponds to a full sequence of the piston's movement through intake, compression, power, and exhaust strokes.