Which power source has been least developed?

wood

wind

biofuel

hydroelectric

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.

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).