In which situation would air density be at its highest?

Air density is fundamentally influenced by altitude, temperature, and humidity. At sea level, the atmospheric pressure is highest, which corresponds to a greater number of air molecules present in a given volume compared to higher altitudes. This means that the air is more compressed and, therefore, denser at lower elevations.

As elevation increases, such as at the peak of a mountain or in an airplane at cruising altitude, the atmospheric pressure decreases and the air expands, leading to lower air density. Similarly, in a desert, although the temperature may be high, the air is generally less dense because of the high altitude and lower atmospheric pressure in those regions.

Consequently, the situation at sea level provides the optimal conditions for the highest air density due to the greater weight of the air column above pressing down on the air below, allowing it to be more compact.