The correct answer is: A. Nuclear power plant.
Nuclear power plants have the highest maintenance costs of all types of power plants. This is because they are complex and require a high level of safety and security. Nuclear power plants also have a long lifespan, which means that they require regular maintenance to ensure that they continue to operate safely and efficiently.
Hydro-electric power plants have lower maintenance costs than nuclear power plants. This is because they are simpler and do not require the same level of safety and security. Hydro-electric power plants also have a shorter lifespan, which means that they do not require as much regular maintenance.
Thermal power plants have maintenance costs that are in between nuclear power plants and hydro-electric power plants. This is because they are more complex than hydro-electric power plants, but they do not require the same level of safety and security as nuclear power plants. Thermal power plants also have a longer lifespan than hydro-electric power plants, but not as long as nuclear power plants.
Diesel engine power plants have the lowest maintenance costs of all types of power plants. This is because they are the simplest type of power plant and do not require the same level of safety and security as other types of power plants. Diesel engine power plants also have a shorter lifespan than other types of power plants, which means that they do not require as much regular maintenance.