Which of the following is a device used for backflow prevention?

A pressure vacuum breaker is a specific device designed to prevent backflow in plumbing systems. Backflow occurs when water flows in the opposite direction than intended, potentially leading to contamination of the water supply. The pressure vacuum breaker operates by breaking the siphon effect that can create negative pressure in a plumbing system, which can pull contaminated water back into the clean supply.

The device includes an air inlet that opens during a drop in pressure, allowing air to enter the system and preventing backflow by equalizing the pressure. This is essential in applications such as irrigation systems where water from the ground may be contaminated.

Other options like water filters, chlorinators, and water softeners serve different purposes. For instance, water filters are used to clean water by removing impurities, chlorinators disinfect water, and water softeners treat hardness in water. None of these devices have the primary function of preventing backflow, which is crucial for protecting potable water supplies from potential hazards.