Which tool is used for finishing off holes to a desired diameter?

The tool used for finishing off holes to a desired diameter is the reamer. A reamer is specifically designed to refine the size and finish of existing holes. Unlike a drill bit, which is primarily used for creating the hole initially, a reamer can create a smooth finish and achieve a more precise diameter.

Reamers are particularly useful when a tighter tolerance is required than what a drill bit can provide. They come in various sizes and types, allowing for adjustment of the hole size with great accuracy.

The other options serve different functions. A drill bit is used for drilling holes and does not provide the precision necessary for finishing. A tapper is used to create threads inside a pre-drilled hole for screws or bolts, while a countersink is utilized to create a conical hole that allows the head of a screw to sit flush with the material's surface. This makes the reamer the appropriate tool for the task of finishing holes to a specified diameter with precision.