The correct answer is: (a) Relative humidity.
Explanation:
Relative Humidity: It is the measure of how much moisture the air currently holds compared to the maximum amount it can hold at a given temperature. When the air contains moisture to its full capacity, the relative humidity is 100%.
Specific Humidity: It is the mass of water vapor per unit mass of air, including the water vapor.
Absolute Humidity: It is the mass of water vapor present in a unit volume of air.
The term "full capacity" of air in terms of moisture generally refers to the point of saturation, which corresponds to 100% relative humidity.