Which cockpit light is powered by the battery bus and can drain the battery even when the master switch is OFF?

The dome light is the correct answer because it is designed to be powered by the battery bus and is capable of drawing power even when the master switch is turned off. This ensures that when the aircraft is parked and the master switch is not engaged, the dome light can still be utilized for purposes like allowing crew movement in dark conditions, providing safety in emergencies, or during pre-flight checks when the aircraft is not operational. While other cockpit lights may have specific functions tied to the overall electrical system of the aircraft, the dome light’s independent operation can lead to unintentional battery drain if left on, underscoring its unique functionality.