What indicates that the ERJ-145 cabin door is locked?

The correct choice is the indicator light, which provides a clear and immediate visual confirmation of the cabin door's status. In the ERJ-145, the door locking mechanism is designed to enhance safety during flight operations, and an illuminated indicator light specifically signals that the cabin door is securely locked. This is crucial for ensuring the safety of passengers and crew, preventing any accidental openings during critical phases of flight.

While the primary flight display conveys various flight parameters and information to the pilot, it does not provide specific details regarding the locking status of the cabin door. The flight control yoke is primarily used for controlling the aircraft's flight path, and does not have any functionality related to the door locking mechanism. A sonic alert could potentially provide audio notifications for different system alerts, but in the case of the cabin door status, an indicator light is the most direct and effective means of conveying whether the door is locked or not.