What is the main reason for using a feathering propeller during engine failures?

The primary reason for using a feathering propeller during engine failures is to reduce aerodynamic drag. When an engine fails, the associated propeller is typically windmilling, which creates significant drag that can adversely affect aircraft performance and control. By feathering the propeller, the blades are rotated to a position where their angle is aligned with the airflow, effectively minimizing resistance. This reduction in drag allows the aircraft to maintain better performance, aiding in glide and control, and can help maximize the distance over which the aircraft can maintain controlled flight until reaching safety or landing.

The other options, while they might seem relevant to performance, do not address the primary concern during an engine failure. Maximizing engine performance would not apply since the engine is inoperative; enhancing fuel efficiency is less critical in this emergency context and quicker engine restart is not typically facilitated by a feathering propeller in a single-engine failure scenario.