Which IFF mode is a four-digit reply used for airspace management and positive control?

The chosen answer is correct because Mode 3/A is specifically designed for air traffic control and airspace management, utilizing a four-digit identity code. This mode allows air traffic controllers to identify an aircraft uniquely within a specified airspace, enabling efficient and safe monitoring of air traffic. The four-digit code is assigned to ensure that each aircraft can be positively identified and tracked, which is crucial for maintaining the safety and order of air operations.

In terms of functionality, Mode 3/A enables standard identification, thus contributing to effective airspace management. It offers responders essential information while enhancing situational awareness and control.

The other modes serve different purposes: Mode 2 is often utilized for military identification and does not focus primarily on airspace management. Mode C provides altitude information in addition to the identification but isn't predominantly a four-digit reply system. Mode 4 is used for secure military communications and involves cryptographic functionalities, which do not pertain to standard airspace management.