The correct answer is: C. Error signal
An error signal is the difference between the desired value of a variable and the actual value of the variable. It is the input to a controller, which uses the error signal to calculate an output signal that will reduce the error.
A servo signal is a type of control signal that is used to control the position, velocity, or acceleration of a mechanical system. It is typically generated by a controller and sent to a servo motor, which then uses the signal to move the system in the desired way.
A desired variable value is the value that a controller is trying to achieve for a particular variable. For example, if the controller is trying to control the temperature of a room, the desired variable value would be the desired temperature of the room.
A sensed signal is a signal that is generated by a sensor and represents the value of a physical quantity. For example, a temperature sensor might generate a signal that represents the temperature of the environment.
In conclusion, the error signal is the input to a controller. It is the difference between the desired value of a variable and the actual value of the variable. The controller uses the error signal to calculate an output signal that will reduce the error.