Final answer:
The IFF/SIF mode that provides automatic reporting of aircraft altitude is Mode C, which transmits the pressure altitude of an aircraft to air traffic controllers for maintaining safe separation.
Step-by-step explanation:
The Identification, Friend or Foe (IFF)/Selective Identification Feature (SIF) mode that provides automatic reporting of aircraft altitude is Mode C. IFF is a radar-based identification system designed for command and control. It allows military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. Mode C specifically is used for the automatic reporting of an aircraft's pressure altitude, which is derived from the aircraft's transponder. When interrogated, the transponder transmits a response that is coded to represent the altitude of the aircraft. This information is critical for air traffic control purposes to maintain safe separation between aircraft.