What type of controller uses feedback to maintain a set level?

The correct answer highlights a controller type utilized within control systems to regulate output by maintaining a desired set level through feedback. Control systems are designed to receive input signals from sensors, compare these signals with a desired set point, and adjust the output accordingly to minimize any difference or error.

This mechanism of continually measuring the output and adjusting it based on feedback is foundational in various applications, whether in maintaining temperature, speed, or other process variables. While the specific controller types mentioned, such as proportional, integral, and derivative, fall under the broader category of control systems, a control system encompasses the entirety of this process, including the feedback loops and mechanisms that ensure the output remains stable at the set point.

The options presented highlight specific types of controllers, but they serve as components of a control system rather than standalone entities capable of sustaining levels on their own. Therefore, recognizing that control systems are broader frameworks that utilize feedback to maintain control over a desired level clarifies why this option is the best choice.