Concept:Atmospheric humidity refers to the amount of water vapour present in the air. A hygrometer is the instrument specifically designed to measure this humidity.Explanation:- A hygrometer works by sensing changes in physical properties (such as electrical resistance or capacitance) as moisture is absorbed from the air.- The other options have different uses: a photometer measures light intensity, and a potometer measures the rate of transpiration (water loss) in plants — neither measures air humidity.- The table below summarises common instruments and their purposes for quick revision.