B. Oxygen is the most plentiful gas about 21% by volume.
Explanation:
Nitrogen is the most abundant gas present in the atmosphere about 78 % by volume while the second most abundant gas is oxygen which is about 21 % in the atmosphere which is used by the living organism for breathing and energy production. Nitrogen is used by the plants for the production of amino acids and enzymes formation. Carbon-dioxide is present in a very low amount about 0.03 % in the atmosphere which is used by the plants during photosynthesis process.
