Which of the following helps prevent cross-connection in a plumbing system?

A vacuum breaker on a hose bib is specifically designed to prevent backflow in a plumbing system, which is essential to stopping cross-connections. Cross-connections occur when there's a potential for non-potable water to mix with potable water, which can pose health risks. The vacuum breaker works by allowing air to enter the system if a negative pressure occurs, preventing the flow of contaminated water back into the drinking water supply.

While other options may seem relevant, they do not specifically address the prevention of backflow in the same way that a vacuum breaker does. For example, a backflow preventer (also mentioned among the options) is indeed a valid method to prevent backflow, but not all plumbing systems or applications require the more complex backflow preventers; whereas, a vacuum breaker offers a more accessible solution for outdoor spigots and hose connections. Covering sinks with lids does not effectively prevent cross-connections in the way that a vacuum breaker would because it does not address pressure variations that cause backflow. Similarly, using only metal pipes does not inherently prevent cross-connections, as cross-connections can occur regardless of the material used in plumbing systems.