Which power source has been least developed?

wood

wind

biofuel

hydroelectric

The correct answer is biofuel, which is fuel originating from living matter, such as agricultural products, thus making it a biological process. Instead, we are more familiar with fuel that derives from geological processes (such as the creation of fossil fuels).

I believe the answer is: Wind

the main reason for this is that it is hard for us to predict the wind current. So even if we develop a good technology to generate power from the wind, it can only be used on several occasion

Because of this, most of researches focus on developing solar energy and biofuel.