Which IFF/SIF mode provides automatic reporting of altitude?

Mode C is the correct choice for automatic reporting of altitude in the Identification Friend or Foe/Secondary Surveillance Radar (IFF/SIF) system. This mode allows an aircraft to transmit not only its identification but also its altitude information to the monitoring radar systems. Mode C transponders continuously send altitude data in feet, which can be utilized by air traffic control and other aircraft for situational awareness, thereby enhancing the safety and coordination of air traffic.

Modes A and B are primarily focused on providing identification information without altitude reporting. Mode A sends a specific identification code, while Mode B is used mainly for military identification purposes. Mode D, while it does provide some advanced features, does not offer altitude reporting in the same way that Mode C does. Therefore, only Mode C fulfills the requirement for automatic altitude reporting.