The correct answer is: A. the amount of moisture it can hold decreases.
When a given amount of air is cooled, the amount of moisture it can hold decreases. This is because the air molecules move more slowly when they are cold, and they can therefore hold less water vapor. This is why condensation forms on cold surfaces, such as windows in the winter.
Option B is incorrect because the absolute humidity is the actual amount of water vapor in the air, and this does not change when the air is cooled. Option C is incorrect because the relative humidity is the amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at that temperature, and this does not remain constant when the air is cooled. Option D is incorrect because the absolute humidity decreases when the air is cooled, not increases.