Define “transponder” in aviation.

A transponder in aviation serves a crucial function by transmitting specific information about an aircraft's location, altitude, and identification to air traffic control and other aircraft. This device enhances situational awareness and enables more effective air traffic management by providing radar systems with vital data that allows for the tracking and identification of each aircraft within a specified airspace.

When an aircraft's transponder is activated, it sends its unique identifier, along with altitude and other parameters, enabling controllers to ensure separation between aircraft and enhance safety during flight operations. This capability is especially important in busy air traffic environments where precise tracking is necessary to avoid collisions and to facilitate efficient routing.

The other options refer to different systems or devices in aviation: measuring altitude is typically done with an altimeter, navigational aids like VOR or GPS provide pilots with guidance, and engine monitoring systems are distinct from transponders, focusing on the performance and health of the aircraft’s powerplant rather than its position in the airspace.