Concept:Relative humidity is the amount of water vapour present in air.
Explanation:A hygrometer is used to measure humidity, which is the water vapour content of air.
Relative humidity is expressed as a percentage of the actual moisture in the air compared to the maximum moisture the air can hold at that temperature.
A hydrometer measures the relative density of liquids, not humidity.
A pyrometer measures very high temperatures.
A manometer measures gas pressure.
Hence, the correct answer is the hygrometer.
Answer:A. hygrometer