What is used to measure the depth of a liquid?

The correct choice for measuring the depth of a liquid is a sounding tape. This tool typically consists of a long, flexible strip marked with measurements, which can be lowered into a liquid until it reaches the bottom. The depth can be read directly from the tape, making it a straightforward and effective method for depth measurement in various applications, such as in wells, tanks, or natural bodies of water.

In contrast, while a liquid gauge might generally refer to devices that measure liquid levels, it doesn't specifically denote the method used for direct depth measurement like sounding tape does. A pressure transducer measures the pressure exerted by the liquid column above it, which can then be used to calculate depth, but this process is indirect and requires knowledge of the fluid density. A flow meter is designed to measure the flow rate of liquids in pipes, not to measure depth directly. Thus, for the specific task of measuring the depth, sounding tape is the most appropriate and practical choice.