Which scenario would require a 'readback' in a CAS operation?

In a Close Air Support (CAS) operation, a 'readback' refers to the process where the recipient of a message repeats the information back to the sender to confirm that it has been accurately understood. This practice is particularly crucial during the briefing of the plan. The reason for requiring a readback during this phase is to ensure that all parties involved have a clear understanding of the objectives, targets, coordination measures, and the overall operational intent.

By having the aircrew read back critical information, it minimizes the risk of miscommunication, enhances situational awareness, and ensures that everyone is aligned on the plan, which is vital for the safety and success of the operation. Since the briefing sets the stage for all subsequent actions, clear communication and confirmation are essential.

In contrast, while readbacks may be useful in various contexts during communications, they are not strictly required for all communications or only in crisis situations, nor are they solely established upon request. Each of those instances lacks the fundamental importance of confirming understanding at the planning stage, which is the primary rationale for the answer.