Which system's failure most directly ensures the propeller moves to feather position?

The oil pressure system is crucial for the propeller's ability to move to the feather position, as the propeller system uses oil pressure to control the pitch of the blades. When the engine fails or oil pressure drops, a spring mechanism takes over, allowing the blades to move into the feathered position which minimizes drag and helps maintain control of the aircraft during a potential engine out scenario.

The feathering process involves the adjustment of the propeller blades to align with the airflow, reducing drag significantly. If there's an issue with the oil pressure system, the necessary maintenance of blade pitch control cannot occur, ultimately leading to a failure in feathering when needed.

In contrast, while the fuel system, electrical system, and ignition system contribute to engine and propeller operation, they do not directly manipulate the blade pitch to feather as the oil pressure system does. Thus, the oil pressure system's role is essential for ensuring the propeller can feather in case of engine failure.