Which of the following is a Scan on Receive Only technique?

The correct choice is the conical scan receive-only technique. In electronic warfare and radar systems, a scanning method refers to how the system searches for signals or targets. A conical scan implies a specific method where the antenna beam is rotated in a conical shape while receiving signals, allowing the system to pinpoint the location of a target without actively transmitting.

This technique is utilized to enhance the detection and tracking of incoming signals. As it is a receive-only procedure, the system can effectively gather information without disrupting the environment with outgoing signals, a crucial aspect in stealth operations or when trying to avoid detection.

In contrast, frequency agility involves changing frequencies rapidly during transmission to avoid jamming or interference, which is more aligned with active scanning and not receive-only. Instantaneous automatic gain control adjusts signal levels in real-time but does not refer to a scanning method. Therefore, the focus on conical scanning as a receive-only technique distinguishes it as the best answer.