In which scenario would it be appropriate to terminate radar service?

The appropriate scenario for terminating radar service is when the aircraft cancels IFR (Instrument Flight Rules) in busy airspaces. When a pilot cancels IFR, they are indicating that they no longer require air traffic control services for navigation or traffic separation, typically transitioning to visual flight rules. This is particularly significant in busy airspaces, where the workload and traffic management are heightened.

By canceling IFR, the pilot assumes responsibility for their navigation and separation from other aircraft, allowing air traffic controllers to efficiently manage their resources and keep the airspace organized. In such cases, it is standard practice for controllers to terminate radar service to the aircraft, reflecting the change in operational status and ensuring that all parties are aware of the pilot's intention to operate independently.

The other scenarios do not warrant termination of radar service. For example, simply flying above 10,000 feet does not affect radar coverage or the requirement for air traffic control services. Losing radar contact, while requiring additional procedures, typically does not immediately lead to termination of radar service; instead, efforts are made to re-establish contact. Lastly, while busy airspaces pose challenges, the cancellation of IFR specifically provides the necessary context for service termination.