What instrument is used to measure fluid, whether liquid or air?

The instrument that is commonly used to measure fluid, whether liquid or air, is the flow meter. A flow meter is designed to quantify the flow rate of a fluid passing through a pipe or channel, making it applicable to both liquids and gases. It measures the movement of the fluid, providing essential data needed for a variety of applications in utilities and maintenance work.

Pressure gauges measure the pressure of gases or liquids within a system but do not specifically quantify the flow rate. Similarly, a hydrometer is used for measuring the specific gravity of liquids, while a pitot tube is specifically designed for measuring fluid velocity in a flow, particularly in air. These instruments serve distinct purposes and do not capture the flow rate measure as effectively as a flow meter.

In summary, the flow meter's functionality in measuring and indicating flow rates is what makes it the correct choice for this question.