What is the primary role of a transponder?

The primary role of a transponder is to identify and track aircraft. Transponders are electronic devices on an aircraft that respond to radar signals by transmitting information back to air traffic control (ATC) and other aircraft. When ATC queries an aircraft's transponder, it sends back a signal that includes a unique code, indicating the aircraft’s identity and its altitude. This allows air traffic controllers to maintain situational awareness of all aircraft in their vicinity, improving safety and efficiency in airspace management.

By contrast, while other options relate to aviation, they do not represent the fundamental function of a transponder. Enhancing fuel efficiency pertains to the aircraft's systems and design rather than the transponder. Navigation assistance is primarily managed through different systems like GPS or VOR, and monitoring engine performance involves separate monitoring devices that track engine parameters. Thus, the transponder’s essential function remains its capability to identify and track aircraft for communication and safety in regulated airspace.