The correct option is B relative humidity
Relative humidity is the amount of water vapour or moisture expressed as a percentage of the amount needed for saturation i.e. the maximum amount of water vapour the air can hold at the same temperature. For example, if one cubic metre of air can hold 25g of water at 28∘C, and the actual mositure content is 20g at 28∘C, then the relative humidity is 80 percent.