Which of the following is untraditional sources of Energy?

Answer: Option D

Coal, petroleum & forest fuel such as wood has been used by humans since ages. They are the direct source of energy hence they were more exploited, whereas biogas is a concept of clean energy. The gas produced from animal & human waste is used in various manners as fuel. Conventional sources were direct fuels whereas biogas is first produced from organic waste & than utilised as fuel.