Which statement describes the relationship between altitude and temperature in terms of air density?

The correct choice illustrates the concept that as altitude increases, particularly in the troposphere, air temperature generally decreases. This decrease in temperature affects the air density, as colder air is denser than warmer air. However, the key aspect to note here is that as altitude continues to increase, although temperature may decrease at first, eventually in the stratosphere, temperatures can begin to rise again.

In a more comprehensive view, at higher altitudes, with the overall trend of decreasing temperature, air molecules are more spread out, leading to a lower density of air. This relationship is critical for understanding aviation and meteorological principles, as lower air density affects aircraft performance, lift generation, and engine efficiency.

Other options do not accurately convey the established principles of thermodynamics and the behavior of gases under changing environmental conditions.