In a closed-loop process control system, what is meant by error?

Correct Answer: Option C — The signal in a controller that is obtained by subtracting the measured value of the controlled value from the set point.

In control theory, error is the mathematical difference between the set point (desired value) and the process variable (measured value). This signal is the fundamental input to the controller, which then calculates the necessary corrective action to minimize the deviation and maintain the process at the desired state.