What type of substance could the term "rarefy" describe when talking about air?

The term "rarefy" refers to the process of making something less dense or less concentrated. When applied to air, it specifically implies a reduction in the number of air molecules in a given volume, which results in a thinner or less dense atmosphere. Therefore, when talking about air, "rarefy" aptly describes a thin atmosphere, where the air has been diminished in density, making it less substantial.

The other options do not align with the meaning of "rarefy." Thicker air and heavy smog suggest increased density, while dense vapor indicates a concentration of particles that contradict the concept of making something less dense. Thus, the correct answer pertains to the thinness of the atmosphere resulting from the process of rarefaction.