Which gauge would be most appropriate for measuring the diameter of a bolt?

The micrometer is the most appropriate tool for measuring the diameter of a bolt because it provides a high level of precision and accuracy, which is essential when working with small dimensions like those of a bolt. Micrometers are designed specifically for measuring small distances, typically within a range of one millimeter to several inches, and can measure with precision to within a thousandth of a millimeter (0.001mm or 1µm), making it ideal for applications where exact measurements are crucial.

While calipers are also capable of measuring the diameter of a bolt, they are typically less precise than micrometers due to their larger measuring range and mechanical limitations. Feeler gauges, on the other hand, are used to measure gaps or clearances and would not be suitable for determining the diameter of a bolt. Plug gauges, while useful for checking hole sizes, do not apply to measuring external diameters like those of bolts. Therefore, the micrometer stands out as the best choice for this specific measuring task.