During a flight, if a pilot needs to change their cruising altitude, what must they do?

Informing Air Traffic Control (ATC) before making a change in cruising altitude is essential for maintaining safety and order in the airspace. This requirement ensures that ATC can manage the air traffic effectively, avoiding conflicts with other flights and ensuring that the aircraft remains within safe operational limits.

When a pilot intends to change their altitude, they must request and receive clearance from ATC to ensure that the new altitude is safe and that there are no other aircraft in that space at that time. This process helps in maintaining the planned routes and prevents any potential hazards that may arise from abrupt altitude changes without proper coordination. Additionally, ATC may provide instructions or traffic advisories related to the change, further enhancing safety.

The other options do not prioritize communication with ATC, which is critical in controlled airspace, and could potentially lead to dangerous situations. Therefore, notifying ATC before changing altitude is not only a procedural requirement but also a critical safety measure in aviation.