Concept:A manometer is the instrument specifically designed to measure the pressure of a gas, especially in a closed container.Explanation:A barometer measures atmospheric pressure, not the pressure of a gas in a vessel.A thermometer measures temperature.A hydrometer measures the relative density of a liquid.A manometer measures the pressure of a gas by comparing it with a liquid column, often mercury.Therefore, the correct option is the manometer.Answer:D. Manometer