Which tool is designed with a closed end for making adjustments to bolts?

The tool specifically designed with a closed end for making adjustments to bolts is the box-end wrench. This design allows the wrench to grip the bolt's head thoroughly, providing a secure fit that reduces the risk of slipping while tightening or loosening the bolt. The closed end encases the bolt head, distributing the force evenly and enhancing torque application, making it particularly effective for use in tight spaces where a standard open-end wrench might not fit or provide adequate grip.

In contrast, while the open-end wrench features a U-shaped opening that fits around the bolt, it doesn’t provide the same level of grip and torque as the box-end. The spanner wrench, often used for adjusting nuts on radius fittings, has a unique design that allows for specialized adjustments but is not designed primarily for general bolt use. Lastly, the hydrant wrench is specifically tailored for use with hydrants and may not be suitable for working on typical bolts or fasteners. Thus, the box-end wrench stands out as the most appropriate tool for the task described.