The correct answer is B) Helical. Helical gearing is best suited for medium-speed main propulsion units in Coast Guard-licensed vessels. Helical gears have teeth that are angled relative to the axis of rotation, which provides a smoother, quieter, and more efficient power transmission compared to spur gears (option D). This makes helical gears a better choice for medium-speed applications, where spur gears may be less suitable due to increased noise and vibration. The other options, hypoid (A) and cyclical (C) gears, are not typically used for medium-speed main propulsion units. Hypoid gears are more commonly used in vehicle differential applications, while cyclical gears are a specialized type of gear not commonly found in marine propulsion systems.