What is a "transponder" used for in aviation?

A transponder is an essential device in aviation primarily used to transmit the aircraft's location and altitude to air traffic control (ATC) and other aircraft. This technology enhances situational awareness in the airspace and facilitates collision avoidance by providing crucial information about an aircraft's position. The transponder responds to interrogation signals from ground radar systems, allowing ATC to track aircraft efficiently and accurately.

In modern aviation, transponders often operate in a way that enables them to transmit a unique code (also known as a squawk code) along with altitude data. This means that not only can ATC see where the aircraft is flying, but they can also identify it in a busy airspace, enhancing safety.

While waypoint navigation, autopilot control, and fuel distribution are important systems in aviation, they do not pertain to the primary function of a transponder. Thus, the choice that correctly identifies its role is the one associated with location and altitude transmission.