The correct answer is: C. oxygen
Oxygen is the major constituent of air, making up about 21% of the atmosphere. It is essential for life, as it is used in respiration to produce energy. Oxygen is also used in combustion, which is the process of burning fuel to produce heat or light.
Carbon dioxide is the second most abundant gas in the atmosphere, making up about 0.04%. It is produced by the respiration of animals and plants, and by the burning of fossil fuels. Carbon dioxide is a greenhouse gas, which means that it traps heat in the atmosphere and contributes to global warming.
Nitrogen is the third most abundant gas in the atmosphere, making up about 78%. It is not essential for life, but it is used by plants to produce proteins and other compounds. Nitrogen is also used in the production of ammonia, which is used in fertilizers and explosives.
Hydrogen is the most abundant element in the universe, but it makes up only about 0.00005% of the atmosphere. It is a flammable gas that is used in rocket fuel and other applications.
In summary, the major constituent of air is oxygen. Carbon dioxide, nitrogen, and hydrogen are also present in the atmosphere, but in much smaller amounts.