The correct answer is: C. Pumped storage plant.
A pumped storage plant is a type of hydroelectric power plant that uses two reservoirs at different elevations to store water. During periods of low demand, excess electricity is used to pump water from the lower reservoir to the upper reservoir. During periods of high demand, the water is released from the upper reservoir to flow through a turbine, generating electricity.
Pumped storage plants are often used to provide peak load power, which is the maximum amount of electricity that is needed at a given time. This is because they can be quickly started and stopped, and they have a high efficiency.
Nuclear power plants and steam turbine plants are not invariably used for peak load power. Nuclear power plants are typically used to provide baseload power, which is the minimum amount of electricity that is needed at all times. Steam turbine plants can be used to provide either baseload or peak load power, depending on the design of the plant.
None of the above is not the correct answer.