Of the following propulsion modes, which one would operate with a geared-drive featuring a double reduction?

Correct Answer: Option D — Gas turbine

Gas turbines operate at extremely high rotational speeds, often exceeding 3,000 to 10,000 RPM. To reduce this to propeller speeds, double reduction gearing is required to achieve the necessary gear ratio in a compact, efficient housing, as single reduction sets would require impractically large gear diameters.