Final answer:
In a non-compensated single-unit, dual-type magneto system of a nine-cylinder radial engine, the distributors will have the lowest RPM at any given engine speed, as they are geared down to match the timing of the engine’s pistons.
Step-by-step explanation:
The part of a nine-cylinder radial engine's magneto system that will have the lowest RPM at any given engine speed is the distributor. In a radial engine with a non-compensated single-unit, dual-type magneto system, the rotating magnet of the magneto always turns at a lower RPM than the engine itself because it is geared down to synchronize the firing of the spark plugs with the position of the pistons. Since the distributors are separately mounted but driven by the magneto, they will rotate at the same lower RPM as the magneto's rotating magnet, which in turn is lower than the engine RPM.