The correct answer is: C. full load
Generators for peak load plants are usually designed for maximum efficiency at full load. This is because peak load plants are only used during periods of high demand, when the demand for electricity is greater than the supply. During these periods, the generators are operated at their maximum capacity, which is when they are most efficient.
The other options are incorrect because they do not represent the most efficient operating point for generators for peak load plants. Option A, 25 to 50 percent full load, is incorrect because generators are not as efficient at these loads. Option B, 50 to 75 percent full load, is incorrect because generators are still not as efficient at these loads. Option D, 25 percent overload, is incorrect because generators are not designed to be operated at overloads.
In conclusion, generators for peak load plants are usually designed for maximum efficiency at full load. This is because peak load plants are only used during periods of high demand, when the demand for electricity is greater than the supply. During these periods, the generators are operated at their maximum capacity, which is when they are most efficient.