The correct answer is: D. Specific humidity
Specific humidity is the ratio of the mass of water vapor to the mass of dry air in a parcel of air. It is often expressed as a percentage.
Absolute humidity is the mass of water vapor per unit volume of air. It is often expressed in grams per cubic meter.
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.
Mixing ratio is the ratio of the mass of water vapor to the mass of dry air in a parcel of air, multiplied by 1000. It is often expressed in grams per kilogram.
Saturation vapor pressure is the pressure exerted by water vapor in a parcel of air when the air is saturated. It is often expressed in millibars.
Specific humidity is the most appropriate measure of the density of water vapor in a given parcel of air because it takes into account both the mass of water vapor and the mass of dry air in the parcel.