Biogas is a type of gas that is produced by the fermentation of biomass. Biomass refers to organic matter such as plant material, animal waste, and agricultural residues. During the fermentation process, bacteria break down the biomass in the absence of oxygen, resulting in the production of biogas. This gas is primarily composed of methane and carbon dioxide, although it may also contain small amounts of other gases such as hydrogen and nitrogen.

Option 1, which states that biogas is a mixture of gases from volcanoes, is incorrect. Volcanic gases typically consist of water vapor, carbon dioxide, sulfur dioxide, and various other compounds, but not biogas.

Option 2, which suggests that biogas is gas produced from certain crude oil wells, is also incorrect. Biogas is distinct from the naturally occurring gas found in crude oil reserves.

Option 3, which states that biogas is gas produced by incomplete combustion of biomass, is incorrect as well. Incomplete combustion of biomass typically results in the production of smoke and pollutants, not biogas.

Therefore, the correct statement is option 4 - biogas is produced by the fermentation of biomass.