Which mode under IFF is primarily used for civilian and military air traffic control?

Mode 3 is the correct choice for identifying the mode used primarily for civilian and military air traffic control. This mode is standardized for transponder operation to be utilized in air traffic control by providing a four-digit code, known as a squawk code, which pilots can set on their aircraft's transponders. Air traffic control utilizes Mode 3 signals to track aircraft, improve situational awareness, and ensure safety in controlled airspaces. The radar systems associated with air traffic control interpret these signals, allowing for effective monitoring and management of both civilian and military air traffic.

Other modes serve different purposes. For instance, Mode 1 is typically used for military purposes and is not standardized for civilian air traffic control. Mode 4 is a secure mode used to identify friendly military aircraft and is primarily utilized during combat conditions. Mode C provides altimeter information, which can be important for operations but is not primarily the mode for identifying aircraft as part of air traffic control. Thus, Mode 3's dedicated function in facilitating the identification and tracking of aircraft by air traffic controllers underlines its central role in both civilian and military aviation.