In an electric control circuit, the sensing unit may also be the:

In an electric control circuit, the sensing unit can serve as the controller. This is because the controller is responsible for taking the input from the sensing unit—such as temperature, pressure, or humidity—and using that information to make decisions regarding the operation of the system.

For instance, if the sensing unit detects a temperature that exceeds a preset limit, the controller processes this information to activate or deactivate the necessary devices within the circuit to maintain the desired temperature. The controller essentially interprets the data from the sensing unit and orchestrates the action required to achieve the set point.

Other choices represent components of a control system but are not typically classified as the primary controller. A final control element is responsible for executing the commands given by the controller, such as turning on a valve or a motor. A thermostat is specifically a type of sensor that may operate as a controller in simpler systems, while a humidistat specifically monitors humidity levels. However, the general term "controller" encompasses a broader range of functionalities, making it the more accurate answer in this context.