Explanation

Biogas produced by anaerobic digestion consists mainly of methane (about 55–65%) and carbon dioxide (about 30–40%), with only trace amounts of nitrogen, hydrogen sulphide and other gases. Methane is thus the largest fraction, followed by carbon dioxide, followed by nitrogen. Option A (Methane > Carbon dioxide > Nitrogen) correctly reflects this composition, while the other options wrongly place nitrogen or oxygen above carbon dioxide.