The curb of a shower receptor should be a maximum of how many inches when measured from the top of the drain?

The maximum height of the curb of a shower receptor, measured from the top of the drain, is established for both safety and functionality reasons. A curb height of 9 inches allows for effective water containment within the shower area, preventing overflow and ensuring proper drainage. It's high enough to provide a barrier against water escaping but not so high that it becomes a tripping hazard or is difficult for users to step over when entering or exiting the shower. This height also accommodates various design features in modern bathrooms while ensuring compliance with plumbing codes.

The other options represent heights that exceed the standard maximum, which can lead to practical issues during use and potential non-compliance with building regulations.