The correct answer is D. 21%.
Air is a mixture of gases, and the percentage of oxygen in air is approximately 21%. The other gases in air include nitrogen (78%), argon (0.9%), carbon dioxide (0.04%), and other gases in trace amounts.
Oxygen is essential for life, as it is used in cellular respiration to produce energy. Nitrogen is a relatively inert gas that helps to dilute oxygen and prevent it from becoming too concentrated. Argon is also an inert gas that is used in light bulbs and other applications. Carbon dioxide is a greenhouse gas that helps to regulate the Earth’s temperature.
The percentage of oxygen in air can vary depending on the environment. For example, the percentage of oxygen in air is lower at high altitudes and higher in forests.