Which fastener is commonly identified by its diameter as M6?

The identification of a fastener by its diameter, such as M6, points specifically to a metric bolt. The "M" denotes that it is a metric fastener, and the number that follows indicates its nominal diameter in millimeters. In this case, M6 means the bolt has a diameter of 6 millimeters.

Bolts are designed for applications that require a secure fastening that withstands shear and tensile forces. They are commonly used in a variety of engineering and construction contexts, making their identification straightforward and essential for ensuring appropriate compatibility with other components.

Other options such as nails, hinges, and gears are not identified by diameter in the same way. Nails are typically classified by length and gauge, hinges are described by their material and dimensions, and gears are identified by features such as pitch, number of teeth, and bore diameter, rather than by a singular diameter measurement. Therefore, M6 specifically refers to a metric bolt, confirming that the correct answer is indeed the bolt.