The correct answer is A) Transfer high-speed gas turbine rotation to low-speed propeller rotation. The main reduction gear in a marine gas turbine propulsion system is designed to transfer the high rotational speed of the gas turbine to the lower rotational speed required by the propeller. Gas turbines typically operate at much higher rotational speeds than marine propellers, so the reduction gear acts as an intermediary to match the speeds and allow the propeller to be driven efficiently. The other answer choices are incorrect because: B) Reducing gas turbine speed to engage the clutch is not the primary purpose of the reduction gear; C) Transferring low-speed gas turbine rotation to high-speed propeller rotation is the opposite of what the reduction gear does; and D) Increasing gas turbine speed to engage the clutch is not the function of the reduction gear.