The transponder transmits _____ when the mode is set to ON.

When the transponder is set to ON, it actively transmits the aircraft's squawk code. The squawk code is a unique four-digit identifier assigned to each aircraft that helps air traffic control (ATC) identify and track the aircraft within a given airspace. This code is essential for situational awareness and maintaining safe distances between aircraft.

While altitude data is indeed transmitted by the transponder when it is in altitude reporting mode, the direct question focuses on what the transponder specifically transmits in the ON mode, which is primarily the squawk code. Other options like GPS coordinates and aircraft identification may refer to different operational contexts or systems, but they do not represent the primary function of the transponder's ON mode. Thus, the correct understanding of the transponder's function clearly aligns with the transmission of the squawk code as a fundamental part of air traffic management.