What is a form of variable resistance that can be found in a piping system?

The correct choice is a control valve, which serves as a form of variable resistance in a piping system by regulating the flow of fluid through it. Control valves are designed to adjust the opening size in response to signals from a controller or operator input, thus varying the resistance against the flow of fluid. By doing so, they can maintain desired system parameters, such as pressure, temperature, or flow rate, ensuring efficient operation within the system.

Control valves can be manually operated or automatically controlled, and they can modulate the flow based on specific requirements. This modulation is critical in many industrial applications, where precise control over the flow rate of liquids or gases is essential for maintaining system performance and safety.

Other options, while relevant to piping systems, serve different purposes. Regulators are primarily used to reduce pressure to a set point and do not vary flow resistance dynamically in the same way that control valves do. Flow meters measure the rate of flow but do not control it, and pressure gauges simply provide a reading of pressure without altering flow conditions. Therefore, the unique ability of control valves to vary resistance and control flow makes them the correct choice in this context.