The correct answer is: C. Feedback
The feedback signal is the signal that is returned from the output of a system to its input. The reference signal is the desired output of the system. When the feedback signal and reference signals are equal, the system is said to be in equilibrium. In this state, the output of the system will be zero.
The input signal is the signal that is applied to the input of the system. The actuating signal is the signal that is used to control the output of the system.
Here is a diagram that illustrates the relationship between the input, feedback, reference, and actuating signals:
[Diagram of a feedback control system]
The input signal is applied to the input of the system. The feedback signal is the signal that is returned from the output of the system to its input. The reference signal is the desired output of the system. The actuating signal is the signal that is used to control the output of the system. The output of the system is the signal that is produced by the system.
The system is said to be in equilibrium when the feedback signal and reference signals are equal. In this state, the output of the system will be zero.