Question

which phrase describes air density? increases as altitude increases equals mass divided by volume pushes molecules in one direction exerts less pressure as it increases

Explanation:

Air density is a measure of mass per unit volume of air, which matches the definition "equals mass divided by volume". Air density decreases with increasing altitude, it does not push molecules in one direction (that describes force/pressure), and higher air density leads to higher air pressure, not lower.

Answer:

equals mass divided by volume