The correct answer is: B. base load plant.
A base load plant is a power plant that is designed to operate continuously at a relatively constant load, regardless of the time of day or the demand for electricity. Nuclear power plants are often used as base load plants because they are very efficient and can operate 24 hours a day, 7 days a week.
A peak load plant is a power plant that is designed to operate during periods of high demand for electricity. Peak load plants are often used to supplement base load plants during times of high demand, such as during the summer months when air conditioning use is high.
A stand-by plant is a power plant that is not normally in operation, but is kept on standby in case of an emergency. Stand-by plants are often used to provide backup power in the event of a power outage.
A spinning reserve plant is a power plant that is in operation, but is not currently meeting the demand for electricity. Spinning reserve plants are used to provide a buffer in case of unexpected changes in demand or outages at other power plants.
In conclusion, nuclear power plants are invariably used as base load plants because they are very efficient and can operate 24 hours a day, 7 days a week.