The correct answer is C. Nuclear power plant.
A base load power plant is a power plant that runs at a constant rate to meet the minimum load on an electrical grid. Nuclear power plants are almost inevitably used as base load plants because they are very efficient and can operate 24 hours a day, 365 days a year. They also have a low environmental impact, which makes them a good choice for meeting the growing demand for electricity.
Diesel engine plants, gas turbine plants, and pumped storage plants are not as well-suited for base load applications. Diesel engine plants are relatively inefficient and expensive to operate. Gas turbine plants are also inefficient, and they can only operate for short periods of time before they need to be shut down for maintenance. Pumped storage plants are used to store energy during periods of low demand and then release it during periods of high demand. They are not used as base load plants because they are not always available to meet the demand.
In conclusion, nuclear power plants are almost inevitably used as base load plants because they are efficient, reliable, and have a low environmental impact.