[A]. the mass of water vapour present in 1 m3 of dry air
[B]. the mass of water vapour present in 1 kg of dry air
[C]. the ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure.
[D]. the ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure
✅ The correct answer is option C.