What is true about a proportional-only controller?

A proportional-only controller is characterized by its ability to adjust the output in proportion to the difference (error) between the set-point and the process variable. However, one key limitation of this type of control system is that it cannot always maintain the process variable exactly at the set-point. This is primarily due to the inherent offset that exists in a proportional-only control; as the error decreases, the output reduces, potentially leading to a steady-state error.

For instance, if the set-point is at a certain temperature but the heating element cannot adjust perfectly to maintain that temperature due to system limitations or external disturbances, the process variable may stabilize near the set-point but will not be able to hold it precisely. This offset is a fundamental aspect of proportional control, as it lacks the necessary integral action to eliminate the steady-state error completely.

In contrast, the other options either describe characteristics that are not applicable to proportional-only controllers or refer to control strategies that incorporate additional mechanisms, such as reset action or on/off control, which are not the primary function of a proportional-only controller.