Which of the following fittings is NOT permitted in the sanitary drainage portion of a DWV system?

In a sanitary drainage portion of a Drainage, Waste, and Vent (DWV) system, certain fittings are specifically designed for effective drainage without hindering flow or creating blockages. The correct answer identifies "2" as a non-fitting and therefore not permitted in this context.

Sanitary tees, wye fittings, and cleanout fittings each serve important functions in a DWV system. A sanitary tee allows the connection of horizontal and vertical pipes without causing significant flow disruption, which is critical to maintain the efficiency of the drainage system. Wye fittings facilitate the branching of drainage lines at shallow angles to ensure smooth flow without creating turbulence, making them suitable for DWV applications. Cleanout fittings provide access to the system for maintenance and cleaning, which is essential to prevent clogs effectively.

Since "2" does not represent any fitting used in plumbing, there is no possibility for it to perform any role within the sanitary drainage system, making it the appropriate choice for the item that is not allowed.