Answer : 2. "Humidity"
Explanation :

Answer: B) Humidity Explanation: The amount of water vapor ranges from a trace amount up to 4% of the mass of air. Water vapor exists as an invisible gas in the air.   The amount of water vapor in the air is called absolute humidity. The amount of water vapor in the air as compared with the amount of water that the air could hold is called relative humidity. The maximum amount of water vapor that can be in the air depends on the air temperature.