What is the primary use of a basin wrench?

The primary use of a basin wrench is to remove nuts and small piping pieces from the underside of sinks and lavatories. This tool is specifically designed to reach tight or awkward spaces, such as those found under sinks, where regular wrenches may not fit. It features a long handle and a pivoting jaw, allowing the user to effectively grip and turn nuts that are often hard to access due to limited space or obstructions.

Other options reflect different tool functions that are not pertinent to the basin wrench. For example, cutting pipe would typically require a pipe cutter or hacksaw, and tightening bolts usually involves a socket wrench or adjustable wrench rather than a basin wrench. Measuring water pressure is a function associated with a pressure gauge or similar devices, which is outside the scope of what a basin wrench is designed for. Thus, option C is the best fit for the specific capabilities of a basin wrench.