Sources of energy can be categorized into 2 categories i.e. commercial and non-commercial. According to you, which of the following is an example of "Non-commercial" source of energy? 

None of the above

There are commercial and non-commercial sources of energy. Commercial sources are coal, petroleum and electricity as they are bought and sold. Non-commercial sources of energy are fuelwood, agricultural waste and dried dung. These are noncommercial as they are not available in the market on a large scale.