The correct answer is: A. 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 a dimensionless quantity, often expressed as grams of water vapor per kilogram of dry air (g/kg).
Mixing ratio is the ratio of the mass of water vapor to the mass of dry air in a parcel of air, multiplied by 100. It is a unitless quantity, often expressed as percent.
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 if it were saturated. It is expressed as a percentage.
Absolute humidity is the mass of water vapor per unit volume of air. It is expressed in grams per cubic meter (g/m3).
Specific humidity is a more useful measure of the amount of water vapor in the air than relative humidity, because it is not affected by changes in temperature. For example, if the temperature of the air increases, the relative humidity will decrease, even though the amount of water vapor in the air remains the same. However, the specific humidity will remain the same, because it is a measure of the mass of water vapor per unit mass of dry air.
Specific humidity is important in meteorology, because it is a measure of the potential for precipitation. The higher the specific humidity, the greater the potential for precipitation.