Concept:Relative humidity is the amount of water vapour present in air compared to the maximum it can hold at a given temperature.
A hygrometer is the instrument designed to measure this quantity.
Explanation:Relative humidity is expressed as a percentage and indicates how damp or dry the air feels.
The hygrometer detects moisture content in the atmosphere.
It commonly uses a wet-bulb and dry-bulb thermometer arrangement or a hair tension mechanism.
Other instruments listed measure different physical quantities.
A hydrometer measures liquid density.
A manometer measures gas or liquid pressure.
A hypsometer measures altitude by using the boiling point of water.
Only the hygrometer is directly used for relative humidity.
Answer:C. Hygrometer