What mode corresponds to the civilian mode A and is manually entered into the transponder by the pilot or aircrew?

Mode 3 corresponds to the civilian mode A and is indeed entered manually into the transponder by the pilot or aircrew. This mode is primarily used for air traffic control purposes, enabling ground radar to identify the aircraft with a unique squawk code assigned to it. When an aircraft is operating in Mode 3, it transmits a four-digit code that air traffic controllers can use to track and communicate with that aircraft, facilitating safe and efficient air traffic management.

The manual entry aspect is significant, as it allows for customization based on flight plans or specific operational requirements, ensuring that each aircraft can be easily seen and identified by radar. This is essential for maintaining order in busy airspace and ensuring that aircraft can be maneuvered safely.

In contrast, the other modes either pertain to different operational contexts or automatic reporting functions, which makes Mode 3 specifically aligned with the question’s requirements.