What condition deactivates autofeather for the left engine when armed?

Autofeather is a safety system designed to improve aircraft handling during an engine failure by automatically feathering the propeller of the inoperative engine, thus reducing drag. In the context of the question, the correct choice indicates that the condition deactivating the autofeather for the left engine occurs when the right engine's torque drops below 410 ft-lb.

This condition is critical because if the right engine experiences a significant loss of power (indicated by the torque drop), it changes the operational dynamics of the aircraft. Autofeather is designed to engage or disengage based on the torque levels of both engines to ensure safety and optimal performance. If the right engine is not providing sufficient thrust, the system recognizes that both engines are not functioning normally, and it may be unsafe to feather one engine while the other is compromised. Hence, deactivating the autofeather for the left engine under this condition is an important safety measure.

In contrast, the other choices do not provide accurate conditions for deactivating the autofeather system. The threshold of 410 ft-lb for the left engine's torque exceeding is a criterion for activating rather than deactivating autofeather, while both engines being idle does not consider the torque levels