What must be provided for every trap serving a lavatory, sink, or laundry tray, except for a floor-mounted service sink?

The provision of a cleanout plug for every trap serving a lavatory, sink, or laundry tray is essential for maintenance and accessibility. Cleanouts are necessary for the removal of blockages that can occur in the trap or the connected piping. If a blockage occurs, having a cleanout plug allows plumbers to easily access the trap to clear any debris or buildup without needing to disassemble the piping. This is crucial for maintaining proper drainage and ensuring that the plumbing system operates efficiently.

Other options such as a removable cover, an additional drain, or a ventilation pipe do not fulfil the same maintenance purpose. While removable covers may assist in access, they do not provide the necessary clear pathway for cleaning blockages within the trap itself. An additional drain is typically not required and could complicate the drainage system further. Meanwhile, a ventilation pipe serves to balance air pressure in the drainage system but does not help in clearing blockages from the traps. Thus, the requirement of a cleanout plug is fundamental in ensuring effective plumbing maintenance and performance.