The correct answer is A. Air is said to be saturated when it contains the maximum amount of water vapor that it can hold at a given temperature and pressure.
Option B is incorrect because air pressure does not affect the amount of water vapor that air can hold.
Option C is incorrect because the thickness of air does not affect the amount of water vapor that air can hold.
Option D is incorrect because air blowing over barren land does not affect the amount of water vapor that air can hold.
The amount of water vapor that air can hold is determined by the temperature of the air. Warmer air can hold more water vapor than cooler air. When the air is saturated, it is holding the maximum amount of water vapor that it can hold at that temperature. If the air is cooled, it will release some of the water vapor as condensation. If the air is heated, it will be able to hold more water vapor.