If a surface finish symbol shows 15 checkmarks, what does it indicate?

The correct interpretation of a surface finish symbol that shows 15 checkmarks is that it indicates a surface finish of 15 microinches. In machinist terminology, checkmarks represent specific values in microinches, which is a common unit of measurement for surface finish in machining processes. This detailed scaling allows machinists to measure and control the roughness of a machined surface to meet specific requirements.

On the other hand, the other options do not accurately correspond to the standard interpretation of surface finish symbols. Microns, millimeters, and nanometers are different units of measurement that would typically use different symbols or indications in the context of surface finish specifications.