What effect does compressing air have on its volume?

Compressing air leads to a decrease in its volume due to the principles of gas behavior and the ideal gas law. When air is compressed, the distance between the gas molecules is reduced because the same number of molecules is being forced into a smaller space. This reduction in space directly results in a decrease in volume.

According to Boyle's Law, which states that the pressure of a gas increases while the volume decreases when temperature is held constant, compressing the air increases its pressure while simultaneously compacting the molecules closer together. This relationship illustrates that when external pressure is applied to a gas, the gas will occupy less volume.

Thus, compressing air indeed decreases its volume, making the answer correct.