The correct answer is option 2, methane. Biogas is predominantly composed of methane (CH4). Methane is a greenhouse gas with a higher global warming potential than carbon dioxide, so its presence in biogas can make it an effective renewable energy source. It can be produced through the anaerobic decomposition of organic matter such as animal manure, plant residues, and food waste.

Let`s analyze the other options:

Option 1: Carbon dioxide (CO2) is also present in biogas but in lower quantities compared to methane. It is a less potent greenhouse gas.

Option 3: Hydrogen (H2) can be present in trace amounts in biogas, but it is not the largest composition.

Option 4: Hydrogen sulfide (H2S) is a byproduct of the anaerobic digestion process but does not make up a significant portion of biogas. Its presence in biogas is generally undesirable and needs to be removed due to its corrosive nature.

Therefore, the correct answer is option 2, methane.