During the process of operating the airstairs, what illuminates to indicate power?

The correct answer identifies the specific light that indicates power when operating the airstairs. The "STAIR OPER light" is designed to illuminate as a direct indication that the airstairs are being operated correctly and that power is available for their function.

In aviation operations, visual indicators are crucial for confirming the status and functionality of equipment. The illumination of the STAIR OPER light signifies that the system is active and that it is receiving power, ensuring that the crew can proceed safely without ambiguity about the operation of the airstairs.

The other options might pertain to different systems or statuses within the aircraft but do not specifically indicate power for the airstairs. Understanding the specific function of each light is essential for flight safety and operational efficiency.