Answer : The correct answer is -
C) mixture.
Air can be described as mixture of gases (such as oxygen, nitrogen, carbon dioxide and several other gases) and certain dust particles along with water vapor.
A mixture is considered as a constituent of two or more particles /substances that are present together in any composition and do not necessarily undergo chemical reaction. The particles of a mixture can be separated by physical means (unlike compound, which contains particles that react chemically and present in a fixed ratio).
Thus, mixture is a scientific term that best describes air.