The depth of a 3/8-inch diameter counterbore can be best measured with which tool?

The depth of a 3/8-inch diameter counterbore is best measured using a depth micrometer because this tool is specifically designed to measure depths with high accuracy. The depth micrometer has a rod that extends down into the counterbore, allowing the user to obtain a precise measurement of the counterbore's depth from the surface of the workpiece.

In contrast, while other tools may be capable of providing a measurement, they do not offer the same level of precision or ease of use for this specific application. A dial indicator is typically used for measuring variations in height or to check the flatness of a surface, rather than measuring depth. Digital and vernier calipers are useful for measuring external dimensions and small depths, but they lack the specific design features that a depth micrometer offers for accurately measuring deeper features like counterbores. Thus, for the task of measuring the depth of a counterbore accurately, the depth micrometer is the most suitable tool.