Where can the shut off valve button be found in the lavatory?

The shut-off valve button in the lavatory is typically located under the sink. This placement is strategic as it allows for easy access in case of a plumbing emergency or maintenance needs. By positioning the shut-off valve here, crew members can quickly secure water flow to the sink and toilet without having to search for it in a less accessible area.

In contrast, while options like above the toilet or near the door might seem plausible, they do not provide the same level of practicality for immediate access to regulate water supply. Similarly, the choice of having it on the wall could lead to difficulties in operating it when quick action is necessary. This makes the under-sink location not only a logical choice but a standard practice in aircraft lavatory designs for efficiency and safety.