Concept:Rainfall is measured using a specialized meteorological instrument called a rain gauge.Explanation:A rain gauge collects falling rain in a calibrated container and measures the depth of water accumulated.This gives the amount of rainfall in millimetres or inches.• A thermometer measures temperature.• A hydrometer measures the density or specific gravity of liquids.• A barometer measures atmospheric pressure.Only the rain gauge is used for rainfall measurement.Answer:A. rain gauge