What tool is small, cylinder-shaped, and comes in various sizes for fitting onto bolts?

The correct choice is a socket, which is a small, cylindrical tool designed specifically for fastening and loosening bolts and nuts. Sockets fit onto a ratchet or a wrench handle, allowing for the application of torque in a controlled manner. They come in various sizes to match the different head sizes of bolts, ensuring a snug fit that minimizes the risk of rounding off the corners of the bolt.

Sockets are particularly advantageous because they can access fasteners in tight spaces, where traditional wrenches might struggle, and they provide even torque distribution across the surface of the bolt head. This makes them a staple in automotive repair and maintenance work.

In contrast, a wrench is a tool that may not be cylinder-shaped and typically requires a more manual grip. An adapter generally serves a different purpose, allowing two tools or components to connect in a compatible way, rather than directly fitting onto bolts. A drill bit, on the other hand, is designed for drilling and does not fit onto bolts. Thus, the socket stands out as the most fitting answer in this context.