Which of the following methods is NOT allowed in Texas for backflow prevention?

In Texas, backflow prevention methods are crucial for maintaining safe drinking water by preventing contaminated water from flowing back into the potable water supply. Among the accepted methods, an Atmospheric Vacuum Breaker is a device that protects against backflow under specific conditions but does not provide complete protection for all situations. This device is suitable primarily for non-pressurized systems and can only be used in situations where there is no likelihood of continuous pressure.

The other methods listed—Pressure Vacuum Breaker, Air Gap, and Reduced Pressure Principle Assembly—are all more reliable and widely accepted for preventing backflow in a variety of scenarios. The Pressure Vacuum Breaker is effective in preventing back siphonage, Air Gap is the most effective form of backflow prevention and involves a physical separation between the water supply and potential contaminants, and Reduced Pressure Principle Assembly offers protection in higher-risk situations where hazardous materials could be involved.

Thus, because of the limitations in its application and reliability, the Atmospheric Vacuum Breaker is not allowed as a stand-alone solution for backflow prevention in Texas, making it the correct answer.