Recall the definitions of mixture types: A homogeneous mixture has a uniform composition throughout, while a heterogeneous mixture has visibly different parts or phases.
Consider the composition of air: it is made up of various gases like nitrogen, oxygen, carbon dioxide, and others, all mixed together uniformly.
Since the gases in air are evenly distributed and you cannot distinguish separate components by the naked eye, air does not have visibly different parts.
Recognize that a compound is a substance formed by chemically bonding elements in fixed ratios, which is not the case for air since its components are physically mixed, not chemically combined.
Conclude that air fits the definition of a homogeneous mixture because it is a uniform physical blend of different gases.