The correct answer is D. output and feedback signal.
A feedback control system is a system that uses feedback to control its output. The feedback signal is a measure of the system’s output, and it is used to adjust the system’s input so that the output meets the desired specifications.
In order for a feedback control system to work properly, the output must be a function of the feedback signal. This means that the output must be affected by the feedback signal. If the output is not affected by the feedback signal, then the system will not be able to control its output.
The other options are incorrect because they do not take into account the feedback signal. Option A is incorrect because the output is not a function of the reference signal. Option B is incorrect because the output is not a function of the input signal. Option C is incorrect because the output is not a function of both the input signal and the feedback signal.