The correct answer is B) Transfer high-speed gas turbine rotation to low-speed propeller rotation. The main reduction gear in a marine gas turbine propulsion system is responsible for converting the high rotational speed of the gas turbine to the lower rotational speed required to efficiently drive the ship's propeller. Gas turbines typically operate at much higher speeds than propellers, so the reduction gear acts as a speed reducer to match the turbine's output to the propeller's needs. This is necessary to efficiently transmit the power from the high-speed turbine to the low-speed propeller. The other answer choices are incorrect because they do not accurately describe the primary purpose of the main reduction gear. Increasing or decreasing the gas turbine speed is not the role of the reduction gear, and it does not transfer low-speed turbine rotation to high-speed propeller rotation.