What must be set on the transponder for secondary radar to receive altitude information?

For secondary radar systems to receive altitude information, the transponder must be set to the mode that transmits altitude data. In this case, the correct setting is Mode C. Mode C is specifically designed to send altitude information along with the aircraft's identity and location to ground-based radar systems.

When an aircraft's transponder is set to Mode C, it broadcasts a signal that includes the aircraft's pressure altitude, which allows air traffic controllers to accurately track the aircraft's altitude. This information is crucial for maintaining safe separation between aircraft and for facilitating efficient air traffic management.

In contrast, other modes such as Mode S are more advanced and can provide additional functionalities, like selective interrogation and enhanced data transmission, but Mode C is the standard setting required for altitude reporting. The Standby mode does not send any information, and ALT is a term that may refer to altitude settings but is not an official transponder mode, thus making Mode C the necessary choice for the transmission of altitude data to secondary radar.