In order to prevent backup of sewage waste in a culinary sink, which of the following must be installed?

To prevent the backup of sewage waste in a culinary sink, it is essential to install an air gap. An air gap is a physical separation between the end of a pipe and the flood level of a sink, which ensures that any potential backflow from a sewage system cannot contaminate the water supply. This gap acts as a barrier against the unwanted reversal of flow, thus protecting the culinary sink from potentially hazardous sewage overflow.

The air gap is a critical component of plumbing systems in food service establishments, ensuring that drinking water remains uncontaminated by wastewater or sewage. It is a straightforward and effective measure to maintain hygiene and safety in culinary environments. Other options, while important in their respective contexts, do not specifically address the prevention of sewage backup in the same direct manner as an air gap. For instance, a backwater valve can help prevent sewage from flowing back into a building, but it does not provide the same level of immediate protection as an air gap against potential cross-contamination at the sink level.