What must be done if the left engine is out and AP/AT is engaged?

The correct response emphasizes the importance of ensuring that the right engine maintains power when the left engine is out and the Automatic Pilot (AP) and Automatic Throttle (AT) systems are engaged. In this situation, maintaining power in the right engine is crucial for several reasons.

Firstly, the aircraft may need to maintain balance and control, and without the left engine contributing thrust, the right engine must compensate to stabilize the flight path. This helps to prevent the aircraft from rolling excessively to the left due to the loss of thrust from the left engine.

Secondly, maintaining adequate power allows the pilot to control the aircraft’s altitude and speed effectively. If the right engine's power is not sustained, the aircraft can experience a loss of control, which could lead to an emergency situation.

Lastly, depending on the aircraft's design and operational protocols, engaging the AP and AT while ensuring the right engine maintains power could provide the necessary assistance to manage the aircraft's flight dynamics during this critical situation. This choice aligns with standard procedures for dealing with engine failures while using automated systems.