If the density of air molecules is more in the given region, then ______.
it is a compression which is formed
If the density of air molecules is more in the given region, then the number of air molecules are close to each other forming a compression or a region of high pressure. So one can say, more is the density more will be the pressure resulting in a compression. Following diagram explains more about sound wave formation due to back and forth motion of tuning fork's prongs, and shows the required high pressure region as well.