The correct answer is B) provide sufficient force to operate large steam lifting beam control valves. In a mechanical-hydraulic governing system on a turbo-generator unit, the pilot valve and servomotor are used to generate the necessary force to operate the large steam control valves, which are typically of the lifting beam type. The pilot valve senses changes in speed or load and signals the servomotor to move the control valves accordingly, allowing the system to maintain the desired turbine speed and generator output. The other options are incorrect because they do not accurately describe the primary function of the pilot valve and servomotor in this type of governing system. Option A is incorrect as the system is not designed to maintain a constant load on the turbine. Option C is incorrect as the parallel operation and speed droop control are handled by other components in the governing system. Option D is incorrect as the pilot valve and servomotor are not responsible for maintaining a constant output voltage, which is typically controlled by the generator's voltage regulator.