Which of the following is needed for automatic track initiation on radar?

Automatic track initiation on radar relies fundamentally on the ability of the radar system to identify and track aircraft effectively. A discrete beacon code is vital for this process because it provides a unique identifier for individual aircraft transponders. This code enables the radar equipment to differentiate between multiple targets in the sky and process them accurately, facilitating the initiation of a track automatically as the radar system receives the signal.

When a radar system detects a transponder signal with a discrete beacon code, it can automatically begin tracking that aircraft without additional input required from air traffic controllers. This capability enhances situational awareness and reduces response time, critical in managing multiple aircraft in busy airspace.

The other options, while related to air traffic management, do not directly support the automatic initiation of tracking in the same way. A unique flight identifier and an active flight plan aid in tracking and coordination but are not necessary for the radar system to establish a track automatically. Similarly, a controller request, although important for operational commands, does not provide the radar with the specific information it needs to begin tracking a target independently.