What tool should be used for backing up the pipe and fittings when assembling a plumbing system?

The most suitable tool for backing up the pipe and fittings when assembling a plumbing system is a pipe wrench. This tool is specifically designed to grip and turn pipes, allowing the plumber to hold one fitting securely in place while tightening or loosening another. The adjustable jaw feature ensures a strong, reliable grip that minimizes the risk of damaging the fittings, which can happen if improper tools are used.

Using a pipe wrench allows the plumber to apply the necessary torque to make effective connections without causing leaks, which is crucial for a plumbing system's integrity. Other tools listed, such as a hacksaw, screwdriver, or scissors, do not provide the same level of support or grip needed for securely managing pipes and fittings; they serve entirely different purposes within the plumbing trade.