Which of these methods is used for backflow prevention?

The air gap is a recognized and effective method for backflow prevention. It creates a physical separation between the outlet of a water supply and the receiving vessel, thereby ensuring that there is no potential for contamination of the water supply. By maintaining this vertical distance, an air gap effectively prevents any backflow from occurring, even during scenarios where there may be a significant drop in water pressure that could otherwise draw contaminated water back into the clean water supply. This method is commonly used in plumbing systems to protect potable water sources from contamination due to backflow incidents.

In contrast, other methods listed don't provide the same level of reliable backflow prevention. Gravity siphons and submersible pumps are typically associated with the movement of water rather than backflow prevention. Pneumatic seals are also not a primary method for ensuring backflow prevention and instead focus more on maintaining pressure and preventing leaks in a more general sense. Thus, the air gap stands out as a definitive solution for preventing backflow, making it the correct choice in this context.