The correct answer is D) provide sufficient force to operate large steam lifting beam control valves. The pilot valve and servomotor in a mechanical-hydraulic governing system on a turbo-generator unit are used to provide the necessary force to operate the large steam lifting beam control valves. These valves are responsible for regulating the flow of steam to the turbine, which in turn controls the power output of the generator. The pilot valve and servomotor work together to rapidly and precisely adjust the valve position as needed to maintain a constant turbine load, which is the primary function of the governing system. The other answer choices are incorrect because they do not accurately describe the primary purpose of the pilot valve and servomotor in this type of governing system. Maintaining constant output voltage (A), allowing parallel operation with zero speed droop (B), and maintaining a constant load on the turbine unit (C) are not the main roles of these components in the mechanical-hydraulic governing system.