Concept:Relative humidity measures the amount of water vapour in air compared with the maximum the air can hold at a given temperature.
The instrument used to measure this moisture content is a hygrometer.
Explanation:Relative humidity is usually expressed as a percentage.
It compares the actual moisture present in the air with the maximum moisture the air can hold at that temperature.
A hygrometer is designed to measure atmospheric humidity or moisture content.
A barometer measures atmospheric pressure.
A hydrometer measures the relative density of liquids.
A manometer measures gas pressure.
Therefore, the instrument that measures relative humidity is a hygrometer.
Answer:A. Hygrometer