When must a pilot receive clearance in relation to frequency assignments?

A pilot must receive clearance before leaving an assigned frequency to maintain proper communication and ensure safety in air traffic control operations. This protocol allows the controller to manage air traffic effectively and ensures that pilots are aware of any changes in their flight situation or assignments.

By adhering to this requirement, pilots avoid potential misunderstandings or loss of contact with air traffic control, which can lead to navigational errors, interference with other aircraft, or hazardous situations. This practice ensures that all parties are synchronized and enhances the overall safety and efficiency of airspace management.

The other options do not cover the importance of communication clearance at the appropriate times during a flight, which could impact a pilot's safety and flight coordination.