Which transponder mode broadcasts additional information such as the aircraft’s altitude?

Mode S is the transponder mode that broadcasts additional information, including the aircraft's altitude. This is significant because Mode S enhances the capabilities of air traffic control. Unlike Mode A, which only provides a unique identifier for the aircraft, and Mode C, which transmits altitude information but does not have the broader functionalities of Mode S, Mode S allows for both the transmission of altitude data and the ability to support data link communications.

In practice, Mode S transponders can also provide information such as aircraft identification, emergency codes, and aircraft-related data to air traffic control, making them essential for modern air traffic management. The broader data capabilities of Mode S are crucial in busy airspace and help improve situational awareness for both pilots and controllers. This capability to transmit altitude information, along with additional data, is what distinguishes Mode S from other transponder modes.