Which of the listed reduction gear sets has the input and output shafts in-line with one another?

Correct Answer: Option D — Epicyclic gearing

Epicyclic or planetary gearing systems utilize a central sun gear, planet gears, and an outer ring gear. This arrangement allows the input and output shafts to remain concentric and in-line, providing a compact design ideal for high-reduction ratios in marine propulsion and auxiliary machinery applications.