Which of the following is a basic backflow prevention device?

The air gap is a fundamental backflow prevention device because it provides a physical separation between the potable water supply and any contaminated source. By creating this gap, it ensures that even in the event of a drop in water pressure, contaminated water cannot siphon back into the clean water system, thus protecting the drinking water from possible contaminants.

Air gaps are widely recognized as one of the most effective methods for backflow prevention and are commonly used in various applications, especially where water safety is critical, such as in kitchens and laboratory settings. The effectiveness stems from the fact that it relies solely on the physical space, making it immune to mechanical failure.

The other options, while they have their uses in various water management systems, do not provide the same level of assurance against backflow. Single check valves and vacuum breakers can prevent backflow to an extent, but they may fail under certain conditions. Ball valves control flow but do not prevent backflow on their own. Therefore, the air gap stands out as the most reliable and basic method for ensuring backflow prevention.