The correct answer is: A. Absolute humidity.
Absolute humidity is the mass of water vapor per unit volume of air. It is often expressed in grams per cubic meter (g/m3) or kilograms per cubic meter (kg/m3).
The maximum pressure that water vapor molecules would exert if the air were saturated is called the saturation vapor pressure. The saturation vapor pressure is a function of temperature and is higher at higher temperatures.
Boiling point is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. At this temperature, the liquid is in equilibrium with its vapor phase and the liquid will boil.
Mixing ratio is the ratio of the mass of water vapor to the mass of dry air in a mixture. It is often expressed as a percentage.
None of the above options are the maximum pressure that water vapor molecules would exert if the air were saturated.