Answer: homogeneous mixture.
Justification:
1) An element is a pure substance constituted by one kind of atom only. For example, iron, oxygen, gold, nitrogen, hydrogen. So, the air is not an element.
There are 118 known elements and you find them in a periodic table.
2) A compound is a pure substance constituted by two or more kind of atoms, in the same fixed proportion. For example, water has always two atoms of hygrogen per each atom of oxygen, that is why its chemical formula is H₂O. Air does not have the same kind of atoms bonded in a fixed ratio. So air is not a compound. Other examples of compounds are: CO₂, CH₄, NH₃. There are infinite different chemical compounds.
3) Homogeneous mixture: A mixture does not have a definite composition. A mixture is composed of two or more pure substances (elements or compounds) in any proportion. Each pure substance keeps its own individual features. The substances that form the mixtures can be separated by physical media.
So, the air is a mixture (of oxygen, nitrogen, carbon dioxide, water vapor, and other gases).
Is it a homogeneous mixture or a heterogenous one?
A homogeneous mixture has a uniform composition (the composition is the same in different parts) and is always is one only phase.
The air that is in a given space (in a house for example) is in the same state (gas) and is approximately uniform in composition (this last is not strictly true, but is very close). So air is generally considered a homogeneous mixture.
Othe examples of homogeneous mixtures are the solutions: a brine, is the most common example.
