Out of the given choices, Hydro-electrical Power Plant, is most Eco-friendly.
Nuclear power plants are having the best productivity ratio for the energy, but it is not eco-friendly. Everybody is just aware of what happened in Japan after the storm tampered the nuclear power plant. The after effects are so horrible, and only god knows how long it will take to nullify the side effects of all the nuclear radiations generated in this incident.
Solar energy and Windmills are also coming up as a good source of energy as it not having any side effects and the basic requirements and available in nature. Solar Energy could be termed as the most efficient and Eco-friendly. Windmill projects are also developing fast in the states of Rajasthan and Maharashtra.
Life is God's precious Gift...!!! Use it to the FULLEST.