A bell is rung. What best describes the density of air around the bell?

The correct answer is D. The air density increases and decreases repeatedly before returning to normal.

When a bell is rung, the sound waves it produced are caused by the vibration of the molecules. Furthermore, the molecules from the metal of the bell continue to vibrate to the air surrounds it. Since the air has a much lower density, the sound can be easily propagated by molecules in it.