A required urinal partition shall extend not less than _____ inches from the wall surface.

The requirement for a urinal partition to extend a minimum distance from the wall surface is based on guidelines set forth to ensure user comfort and privacy. The correct choice, which states that the partition must extend not less than 18 inches from the wall, is aligned with the standard practices in plumbing code. This distance helps to effectively separate the urinal from adjacent structures, helping to create a more spacious and private setting for users.

By having the partition extend at least 18 inches from the wall, it addresses concerns regarding splash-back and ensures better hygiene. Additionally, this distance is sufficient to accommodate most users, making it a functional and practical requirement. Adhering to these measurements is important for maintaining safety and satisfaction in public restrooms.