The percentage of oxygen in air is:

Explanation:

The percent of oxygen in air is 21% in air. It is one of the most important gasses required for the respiration process and hence important for the survival of life on the Earth.

Air is a mixture of different gasses, it constitutes about 78% of Nitrogen, 21% oxygen, and 0.93% of Argon. It also contains a trace amount of other gasses like carbon dioxide, neon, hydrogen.

Thus, option A, B and D do not hold true.

Final Answer:

Hence, the correct option is C i.e., 21%