If a mechanic needs to measure the diameter of a hole approximately 1/4 inch in diameter, which tool should they use?

Using a small-hole gauge is the most appropriate choice for measuring the diameter of a hole that is approximately 1/4 inch in diameter. This tool is specifically designed to accurately measure the internal dimensions of small holes or cavities, making it ideal for this application. The design of a small-hole gauge allows the mechanic to gauge the size of the hole directly, ensuring a precise measurement.

Small-hole gauges typically consist of a pair of tapered, spring-loaded pins that can be inserted into the hole. Once the pins are fully inserted and in contact with the interior surfaces, the mechanic can then remove the gauge and measure the span between the tips of the pins using a caliper or micrometer. This method provides a reliable way to ascertain the hole's diameter, particularly when the opening is too small for conventional measuring tools.

Other tools in the list might not be suitable for this specific task. Calipers, for example, may not offer the accuracy needed for very small openings, and while they are versatile, they are not specialized for internal measurements like those of small holes. A Vernier scale is primarily a scale for comparing measurements and might not be practical for internal diameters without an appropriate attachment. A depth gauge is designed to measure depths rather than diameters,