The correct answer is: A. Nuclear power plants.
Nuclear power plants are complex and require a high level of technical expertise to operate safely and efficiently. The operators of nuclear power plants must have a deep understanding of nuclear physics, reactor design, and plant operations. They must also be able to respond quickly and effectively to any unexpected events.
Gas turbine power plants are also complex, but they do not require the same level of technical expertise as nuclear power plants. The operators of gas turbine power plants must have a good understanding of thermodynamics, fluid mechanics, and combustion. They must also be able to operate the plant safely and efficiently.
Solar power plants are relatively simple to operate. The operators of solar power plants must have a good understanding of solar energy and photovoltaics. They must also be able to operate the plant safely and efficiently.
Hydro-electric power plants are also relatively simple to operate. The operators of hydro-electric power plants must have a good understanding of hydraulics and electricity. They must also be able to operate the plant safely and efficiently.
In conclusion, nuclear power plants require the highest level of technical expertise to operate safely and efficiently. Gas turbine power plants, solar power plants, and hydro-electric power plants are all less complex and require a lower level of technical expertise to operate.