Which physical activity is least affected by changes in air density?

Sitting is the physical activity least affected by changes in air density because it involves minimal movement and does not rely on aerodynamic principles or the influence of wind resistance. When someone is sitting still, the effects of air density changes are negligible, as there is no exertion against the air or aerodynamic drag to consider.

In contrast, activities such as cycling, skydiving, and running involve significant movement through the air, making them much more dependent on air density and atmospheric conditions. For example, in cycling, a cyclist encounters air resistance while pedaling, which can increase with lower air density, as less air provides less resistance. Similarly, during skydiving, a person's fall is influenced by air density; as they drop, they experience varying rates of speed and drag based on how dense the air is. Running also encounters air resistance, affecting speed and performance as air density changes.

Because sitting does not involve these dynamics, it remains largely unaffected by variations in air density, making it the correct answer.