The correct answer is D) Medium-speed, reversing engines. Diesel engines powering large vessels that drive fixed-pitch propellers through a non-reversing reduction gear are considered medium-speed, reversing engines. This is because the engine itself is classified as a medium-speed engine, typically operating between 300-900 rpm, and the reduction gear allows the propeller to be reversed, even though the engine itself does not reverse. The other options are incorrect because: A) Non-reversing engines cannot change the direction of the propeller; B) Slow-speed engines typically operate below 300 rpm; and C) Slow-speed, non-reversing engines lack the ability to reverse the propeller.