C. Reducing the air pressure allows the molecules to escape.
Explanation:
Lower air pressure can be found in the mountain or any high ground place. In this place, you will need less temperature to boil water or other liquid.
A liquid phase will change into gas when it reaches the vapor pressure of the surrounding air. At that point, the molecule can move freely as the gas phase. Since the atmospheric pressure is preventing the liquid to become gas, if the pressure is lower, the liquid can boil at a lower temperatures.
