The mass of water vapour in a unit mass of air is called as:

Correct option is B

Specific Humidity:
· It is defined as the mass of water vapour present in a unit mass of moist air (dry air + water vapour). It is expressed in terms of mass ratio​​.

Key Distinctions:
· Absolute Humidity: The mass of water vapour per unit volume of air (e.g., grams per cubic meter).
· Mixing Ratio: The ratio of the mass of water vapour to the mass of dry air, not including the mass of water vapour itself.
· Saturation Mixing Ratio: The maximum possible value of the mixing ratio when the air is saturated with water vapour at a given temperature and pressure.
Specific humidity is a widely used metric in meteorology and is preferred because it remains nearly constant regardless of changes in pressure or temperature.