The purpose of the main reduction gear in a marine gas turbine propulsion installation is which of the following?

Correct Answer: Option B — Transfer high-speed gas turbine rotation to low-speed propeller rotation.

The primary function of the reduction gear is to bridge the gap between the high-speed output of the gas turbine and the low-speed requirements of the propeller. This allows both the engine and the propeller to operate within their respective optimal efficiency ranges.