The correct answer is: A. feedback
Feedback is any externally introduced signal affecting the controlled output. It is used to control the output of a system by comparing it to a reference signal and adjusting the input accordingly.
Stimulus is a change in the environment that causes a response from an organism. It can be physical, chemical, or biological.
Signal is a physical quantity, such as voltage or current, that carries information. It can be analog or digital.
Gain control is a mechanism that adjusts the gain of an amplifier. It is used to control the amplitude of the output signal.
In the context of the question, feedback is the most appropriate answer because it is the only option that refers to a signal that affects the controlled output. The other options refer to signals that affect the input or the system itself, but not the controlled output.