What would increase the density of air acting on an aircraft?

Increasing the density of air acting on an aircraft is primarily associated with a decrease in altitude. As altitude decreases, the air pressure increases, which in turn increases the density of the air. This occurs because, at lower altitudes, there are more air molecules present in a given volume due to the greater atmospheric pressure. Higher density air provides more lift for the wings of an aircraft and better engine performance, which is beneficial for flight operations.

While an increase in altitude results in a decrease in air density, an increase in air temperature also leads to lower density since warm air expands and occupies more volume. Additionally, decreasing static pressure typically corresponds with increasing altitude (where pressure is lower) and can also decrease density. Therefore, the only condition among the options provided that definitively leads to an increase in air density is a decrease in altitude. This principle is a fundamental aspect of understanding the behavior of aircraft in different atmospheric conditions.