The correct answer is B) governor flyweight motion acting on fuel controls through suitable linkage. The direct acting mechanical governor used with small diesel engines controls the fuel flow to the engine by using the governor flyweight motion to directly manipulate the fuel controls through a mechanical linkage. The flyweights move outward as engine speed increases, and this motion is translated through the linkage to reduce the fuel supplied to the engine, thereby controlling its speed. This is the fundamental mechanism by which a direct acting mechanical governor regulates the engine's fuel delivery. The other options are incorrect because they describe different governor mechanisms: A) uses a pilot valve, C) uses a butterfly valve, and D) uses a servomotor piston, which are not characteristic of the direct acting mechanical governor.