Question

what causes differences in air pressure?
the horizontal movement of air
the speed at which wind moves
the unequal heating of earths atmosphere

Explanation:

Differences in air pressure are mainly caused by the unequal heating of Earth's atmosphere. Uneven heating leads to variations in air density and temperature, creating areas of high - and low - pressure. Horizontal air movement and wind speed are results rather than causes of air pressure differences.

Answer:

the unequal heating of Earth's atmosphere