All tubing used for tail pieces in commercial installations shall be _____________ gauge brass.

The correct choice indicates that all tubing used for tail pieces in commercial installations is required to be 17 gauge brass. This specification is vital because it ensures adequate strength and durability of the tail pieces, which serve as critical components in the plumbing system. Tail pieces need to withstand the operational stresses of water flow and connections to fixtures without failing or leaking.

Using 17 gauge brass provides the necessary thickness and durability to handle various commercial applications, which often involve higher traffic and more robust usage conditions compared to residential settings. The gauge measurement reflects the metal's thickness; therefore, 17 gauge strikes a balance between flexibility for installation and sturdiness for long-term use.

The other gauge options (20, 22, and 18) do not meet the standard required for commercial installations. Lower gauges indicate thicker materials, which offer enhanced durability critical in commercial environments. This makes the 17 gauge choice not only the most suitable but also compliant with the relevant plumbing codes, reinforcing the integrity of the overall system.