D. It is the general term used to describe the amount of water vapor in the air
Explanation:
It's been a while since I've taken meteorology (2 years), but I think this is the correct answer because humidity in general refers to the amount of water vapor in the air and as we are discussing relative humidity, it cannot indicate the actual quantity of water vapor in the air (thus ruling out B). Therefore, D makes the most sense. I believe C refers to adiabatic cooling.
