A portable water supply is physically connected to an automatic fire sprinkler with a fire department connection. What can be used to protect the water supply from backflow

While an atmosphere-type vacuum breaker can prevent backflow by breaking the vacuum that may occur in the system, it is not designed for situations involving continuous pressure and is less reliable under varying conditions. It is more suited for non-pressurized applications and does not provide the same level of protection as a reduced pressure backflow preventer.

The pressure-type vacuum breaker is effective in preventing backflow during periods of low pressure; however, it requires a continuous pressure to function properly. Its application is limited compared to a reduced pressure backflow preventer, which operates effectively even when the water supply pressure varies.

An air gap is a physical separation between the water supply and potential contaminants, effectively preventing backflow. While it is a reliable method, it may not be practical in all situations, especially in systems that need to maintain a constant supply, like fire sprinklers. Therefore, it is not the most suitable option for this scenario.