The correct answer is: Nuclear.
Nuclear power plants have the lowest operating costs of any type of power plant. This is because they are very efficient at converting fuel into electricity, and they do not require much maintenance. Nuclear power plants also have a very long lifespan, which means that they can generate electricity for many years without having to be replaced.
Hydroelectric power plants have the second lowest operating costs. This is because they use the power of moving water to generate electricity, which is a very efficient process. However, hydroelectric power plants can only be built in certain locations, such as near rivers or waterfalls.
Thermal power plants have the third lowest operating costs. This is because they use fossil fuels, such as coal or natural gas, to generate electricity. Fossil fuels are relatively inexpensive, and thermal power plants are relatively efficient at converting fuel into electricity. However, thermal power plants emit greenhouse gases, which contribute to climate change.
Diesel power plants have the highest operating costs. This is because they use diesel fuel, which is a relatively expensive fuel. Diesel power plants are also less efficient at converting fuel into electricity than other types of power plants. Additionally, diesel power plants emit more pollutants than other types of power plants.