What indicates the completion of the air supply inspection?

The conclusion of the air supply inspection is marked by the action of turning off the bus and activating the master switch. This procedure is essential because it allows the driver to confirm that the air supply system is functioning correctly. When the master switch is engaged, it powers the essential systems that rely on the air supply, ensuring that the air pressure builds to the appropriate levels needed for safe bus operation. This verification is crucial for assessing whether the air system can maintain adequate pressure and function properly throughout the vehicle's operation.

The other choices, while related to bus operation and safety, do not specifically conclude the air supply inspection. Turning off the engine and removing the key would signify a departure from the operational phase but does not specifically indicate that the air supply has been checked or ensured. Checking the air gauge for proper levels is a critical part of the air supply inspection but does not represent the completion of the process itself. Ensuring all lights are operational pertains to a different aspect of safety checks unrelated to the specific air supply inspection.