Which valve is primarily used to control water flow in a plumbing system?

The shut-off valve is primarily designed to control the flow of water in a plumbing system. Its main function is to provide a means to stop the flow of water, enabling maintenance or emergency repairs without having to drain the entire system. These valves can be located at various points in the plumbing system, such as under sinks, near water heaters, or at the main supply line to a building, allowing for localized control of water supply.

While other valves mentioned in the options serve specific purposes, they do not primarily focus on controlling the general water flow. The pressure-reducing valve is used to maintain a lower pressure in the plumbing system, preventing damage from high pressure but not directly controlling the flow. The backflow preventer is crucial for preventing contaminated water from flowing back into the clean water supply but does not manage flow rates. An automatic valve can regulate flow based on certain conditions but is more complex and often controlled by sensors or timers, making it not the primary means for manual control of water flow.