Answer: They are all classified as pure substances.
Explanation:
Element is a pure substance which is composed of atoms of similar elements.
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.
Mixtures are heterogeneous materials as they do not have uniform composition and the components are not chemically bonded and not evenly distributed throughout the material.
Air is made up of two elements oxygen
and nitrogen
and a compound carbon dioxide
which are not chemically bonded to one another and thus together form a mixture.