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).
Mixing ratio is the mass of water vapor per unit mass of dry air. It is often expressed in grams per kilogram (g/kg) or kilograms per kilogram (kg/kg).
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor that the air could hold at the same temperature and pressure. It is often expressed as a percentage.
Dew point is the temperature at which the air becomes saturated with water vapor and condensation begins. It is often expressed in degrees Celsius (°C) or degrees Fahrenheit (°F).
Absolute humidity is a measure of the actual amount of water vapor in the air. Mixing ratio is a measure of the relative amount of water vapor in the air. Relative humidity is a measure of how close the air is to being saturated with water vapor. Dew point is the temperature at which the air becomes saturated with water vapor and condensation begins.