Question

The gas, which is present the most in air, is_______________
(A) Oxygen
(B) Nitrogen
(C) Carbon dioxide
(D) Water vapor

Answer 1

Hint : Air is composed of a mixture of gases and is not a gas in itself. Gases such as carbon dioxide, nitrous oxide, methane, and ozone are detectable gases that form about ten percent of the atmosphere. Other components of air are water vapor and dust particles.

Complete step by step solution:

Name of constituents	% Composition
Nitrogen gas	78.1
Oxygen gas	20.9
Carbon dioxide gas	0.03
Argon	0.9

The above list is in descending order of various gases. According to the list, apparently nitrogen is the gas in the air.
Hence option B is correct
Additional Information: The atmosphere is centered on the Earth's surface and you move rapidly, blending with space about 100 miles above sea level. The Earth is actually much thinner than the size of the Earth, equal to the thickness of a piece of paper placed on a beach ball. However, it is responsible for making our earth habitable and producing seasons.
The atmosphere is made up of a mixture of many different gases in varying amounts. Permanent gases whose percentage does not change from day to day are nitrogen, oxygen and argon. Nitrogen is 78%, oxygen 21% and argon 0.9% in the atmosphere. Gases such as carbon dioxide, nitrous oxide, methane, and ozone are detectable gases that form about ten percent of the atmosphere. Water vapor is unique in that its concentrations vary from 0–4% of the atmosphere depending on where you are and what time of day it is. In cold, arid regions, water vapor typically accounts for less than 1% of the atmosphere, while water vapor in moist, tropical regions may account for about 4% of the atmosphere. The amount of water vapor is very important in weather forecasting.

Note: Nitrogen is the most abundant gas in the atmosphere because most of its forms are volatile, doesn’t react with the materials that make up the solid earth and the presence of solar radiation makes it stable.
If oxygen had been the most abundant gas then the atmosphere would have become too poisonous to breathe and if carbon dioxide had been the most abundant gas that would make earth’s atmosphere similar to that of venus.