Master Plumber Practice Exam 2025 - Free Plumber Practice Questions and Study Guide

The correct answer is based on plumbing codes and standards that specify the minimum size for a tailpiece connected to a lavatory. A tailpiece serves as a vertical pipe that connects the sink's drain to the trap, allowing wastewater to flow out of the lavatory.

In most plumbing regulations, the minimum diameter for the tailpiece of a lavatory is specified to be 1-1/4 inches. This size is adequate to ensure proper drainage and minimizes the risk of clogs that can occur with smaller diameters. A 1-1/4 inch tailpiece strikes a balance between sufficient flow capacity and the physical constraints often found in residential plumbing systems.

Other sizes, such as 1 inch, and 1-3/8 inches, do not meet the minimum requirements established in these codes and may lead to problems with drainage efficiency or increased likelihood of blockages. While a 1-1/2 inch tailpiece would also be acceptable, the question specifically asks for the minimum size, making 1-1/4 inches the correct response.