Air content ($a_c$) is the ratio of volume of air to volume of voids.

Degree of saturation ($S_r$) is the ratio of volume of water to volume of voids.

The sum of air content and degree of saturation always equals 1.

Air content is not to be confused with 'percentage air voids' ($n_a$), which relates $V_a$ to the total volume of soil $V$.

Simplifies phase volume calculations

Provides a quick check for soil saturation states

Compaction control of earth embankments

Determining air-void ratios in unsaturated soils

For fully saturated soil, $S_r = 1$, thus $a_c = 0$.

For completely dry soil, $S_r = 0$, thus $a_c = 1$.

Option A is incorrect as it implies air content increases with saturation.