The correct answer is C) The error detector computes the difference between the measured value of the controlled variable and the desired value (set point). In a closed-loop process control system with negative feedback, the error detector is responsible for comparing the measured value of the controlled variable to the desired value (set point). Its function is to compute the difference between these two values, which is known as the error signal. This error signal is then used by the controller to make adjustments to the manipulated variable in order to minimize the difference and bring the controlled variable to the desired set point. The other options are incorrect because they do not accurately describe the function of the error detector in a negative feedback control system. The error detector does not compute the sum, quotient, or product of the measured and desired values; it specifically calculates the difference between them to generate the error signal.