What is the purpose of a transponder in an aircraft?

The purpose of a transponder in an aircraft is to transmit an aircraft's location and altitude to air traffic control. This essential equipment allows air traffic controllers to identify and track the aircraft in real time, enhancing safety and efficiency within controlled airspace. The transponder responds to radar signals from ground stations by sending back information about the aircraft’s position, altitude, and certain operational status, which is crucial for maintaining safe distances between aircraft.

Options related to monitoring fuel levels, providing entertainment systems, or measuring external temperature do not align with the primary functions of a transponder. Instead, they pertain to other systems within the aircraft that serve different operational and comfort-related purposes. The transponder's role is specifically tied to communication and tracking, pivotal components of modern aviation safety and airspace management.