What tool is commonly used to countersink holes?

A countersink bit is specifically designed to create a conical recess around the top of a hole, allowing for a screw to sit flush with or below the surface of the material. This tool achieves this by removing material in a conical shape, which not only accommodates the screw head but also provides a neat finish. This is particularly important in applications where aesthetic considerations are key or where the screw heads should be less obstructive.

While a drill bit is used to create the initial hole and a reamer is intended for resizing and smoothing existing holes, they do not create the conical shape necessary for countersinking. A chamfering tool also provides an angled edge but is not designed specifically for countersinking; its primary function is to bevel edges rather than forming a recess for screws.

Thus, the countersink bit stands out as the correct tool for this specific application, maximizing both functionality and finish quality.