Which of these activities is influenced by increased altitude and decreased air density?

Increased altitude and decreased air density have a significant impact on activities that involve physical exertion, particularly those taking place at high elevations, such as mountain climbing. At higher altitudes, the air pressure drops, which results in lower oxygen levels. This can affect a climber's physical performance, leading to quicker fatigue and requiring acclimatization to avoid altitude sickness.

In contrast, activities like swimming, driving, and fishing are less influenced by altitude and air density. Swimming takes place in water and is not affected by air pressure changes in the same way, while driving and fishing activities do not involve significant physical exertion in higher altitudes that would be impacted by decreased air density. Therefore, mountain climbing is the activity most directly influenced by changes in altitude and air density.