To prevent backflow at the hose attachment of an equipment sink, which of the following must be installed?

The installation of a hose-bib vacuum breaker is crucial for preventing backflow at the hose attachment of an equipment sink. This device specifically helps to stop the backflow of contaminated water into the clean water supply, which can occur when there is a drop in pressure within the water supply system.

When water flows through a hose that is submerged in potentially contaminated water, such as that in a sink, a vacuum can form that pulls non-potable water back into the clean water line. The hose-bib vacuum breaker is designed to break this vacuum, allowing air to enter and thus preventing the backflow of contaminated water.

While other devices like check valves and backflow preventers also play critical roles in preventing backflow in various situations, a hose-bib vacuum breaker is specifically intended for use at hose attachments, making it the best choice for this particular scenario. An air gap is another approach to backflow prevention, involving a physical separation between the water supply and any potential contaminants, but it is not installed directly on hose attachments. Therefore, in this context, the hose-bib vacuum breaker is the most appropriate device to prevent backflow at the hose attachment of an equipment sink.