Concept:A hygrometer is an instrument that measures the moisture content or humidity of the air.Explanation:The hygrometer detects the amount of water vapour present in the atmosphere.It expresses this measurement as relative humidity, which is the percentage of moisture in the air compared to the maximum it can hold at a given temperature.Common examples of hygrometers include the wet and dry bulb psychrometer and the mechanical hygrometer.Therefore, it does not measure specific gravity, rainfall, or salinity.Answer:A. Relative Humidity