Which of the following is a common type of backflow preventer?

A pressure vacuum breaker is a common type of backflow preventer used in plumbing systems to protect potable water supplies from contamination. It operates by utilizing a spring-loaded check valve that opens to allow water flow during normal conditions, but if a drop in pressure occurs, the valve closes to prevent backflow. This mechanism effectively stops contaminants from entering the safe water supply, which is critical for maintaining public health and safety.

Other options listed function differently within plumbing systems. A water filter purifies water but does not prevent backflow; a thermostatic mixer regulates water temperature and does not address backflow issues; and a drain trap prevents sewer gases from entering living spaces but is not designed as a backflow prevention device. Thus, understanding the specific function and purpose of these devices clarifies why the pressure vacuum breaker is the appropriate choice for preventing backflow.