Which tool is most appropriate for loosening or tightening fasteners with a square head?

The most appropriate tool for loosening or tightening fasteners with a square head is the box wrench. A box wrench, also known as a socket wrench, is designed to completely encase the head of a fastener, providing a secure grip that minimizes the risk of slippage while applying torque. This feature is particularly beneficial for square heads, which can otherwise be difficult to grip with tools that do not provide full enclosure.

The design of a box wrench allows for better torque application compared to other tools, making it ideal for fasteners that require a specific amount of force without rounding off the corners. Utilizing a box wrench enables the user to work effectively and efficiently, ensuring that the fastener is either loosened or tightened as required.

In contrast, other tools like the open-end wrench offer only partial contact with the fastener, making them less effective for square heads. The pipe wrench is specialized for gripping round shapes, such as pipes, and is not suitable for fasteners. A torque wrench is specifically used for applying a precise amount of torque to fasteners, but it does not provide the same level of grip and support as a box wrench when it comes to square heads. Thus, the box wrench is the clear choice for this task.