What device is used to maintain a quantity at a desired amount?

A control system is designed to maintain a system's output at a desired level by automatically adjusting the input or conditions based on feedback from the current state. It uses various control strategies to ensure that the system can respond to changes and disturbances in the environment, thereby achieving and maintaining a target value.

In practice, a control system can come in the form of a mechanical, electrical, or digital setup, where sensors measure the current state of the system, and actuators make necessary adjustments to keep the output stable. This dynamic functionality is crucial in various applications, such as temperature regulation in HVAC systems, speed control in motors, and process control in manufacturing.

Other options listed, like a monitoring system, while important for observing parameters, do not actively modify or adjust the system's behavior. A feedback loop is a key component within a control system that provides the necessary input to make adjustments but does not encompass the full functionality of maintaining a desired quantity on its own. The gate controller, while useful in specific scenarios like access management or traffic control, doesn't inherently manage a quantity or maintain a desired set point in the way a control system does. Hence, the control system is the most appropriate choice for this context.