The correct answer is D.
A load curve is a graph that shows how much electricity is being used at a particular time. It is used to help decide the total installed capacity of a plant, the sizes of the generating units, and the operating schedule of generating units.
The total installed capacity of a plant is the maximum amount of electricity that the plant can generate. It is important to choose the right total installed capacity so that the plant can meet the demand for electricity without overloading.
The sizes of the generating units are also important. The generating units should be sized so that they can meet the demand for electricity at all times. However, they should not be too large, as this would be expensive.
The operating schedule of generating units is also important. The generating units should be operated in a way that minimizes costs and maximizes efficiency. The load curve can help to determine the best time to operate each generating unit.
In conclusion, a load curve is a valuable tool that can be used to help decide the total installed capacity of a plant, the sizes of the generating units, and the operating schedule of generating units.