Which grit abrasive is specifically used to reduce the thickness of surfaces?

The correct choice for the grit abrasive specifically used to reduce the thickness of surfaces is 100 grit. This grit is considered coarse and is effective for removing material quickly. It is typically used in initial filing and shaping processes for nails, as it allows for significant thickness reduction in a relatively short amount of time.

Using a coarser grit helps to smooth out uneven areas and quickly shape the nails, making it an essential tool in nail services that require significant adjustments. In contrast, finer grits, such as 180 grit and higher, are more suitable for smoothing out the surface and refining the shape after the thicker material has been removed, focusing more on finishing rather than substantial reduction.