Walk through what anaerobic bacteria actually produce when they digest organic waste without oxygen.
Option A, carbon dioxide: this gas is definitely formed, it is the second most abundant gas in biogas, roughly a third of the mixture, but it is a byproduct, not the major or the useful component, so it is not the best answer.
Option B, methane: methanogenic bacteria are the final and dominant organisms in the anaerobic digestion chain, and their main product is methane gas, making up more than half, usually 55 to 65 percent, of the total biogas produced. This is also the component that burns and gives biogas its use as cooking or lighting fuel, so it is the major constituent by definition and by practical importance.
Option C, hydrogen: hydrogen can appear briefly as an intermediate in the fermentation pathway but methanogens quickly consume it to make methane, so very little hydrogen survives in the final gas mixture, it is not a major constituent.
Option D, nitrogen dioxide: this compound is not generated by anaerobic bacterial digestion at all, it belongs to combustion and atmospheric chemistry, not to biogas production, so it does not belong in this list as a real constituent.
Methane clearly dominates both in quantity and in importance, confirming option B.