Which tool is commonly used to measure the pressure of a fluid?

The manometer is specifically designed to measure the pressure of fluids, making it the correct choice. It operates based on the principle of balancing the fluid pressure against a column of liquid, usually mercury or water. This allows for precise pressure measurements, whether for gas or liquid applications, by indicating the difference in height of the liquid in the two arms of the U-shaped tube.

In contrast, a thermometer is used for measuring temperature, which is not relevant to pressure measurement. A flowmeter measures the flow rate of fluids, which is distinct from measuring pressure. A multimeter is an electrical testing tool that can measure voltage, current, and resistance but does not measure fluid pressure. Hence, the manometer is the appropriate tool for this specific application.