Any externally introduced signal affecting the controlled output is called a

Answer: Option B

Any externally introduced signal affecting the controlled output is referred to as a stimulus. In the context of control systems and feedback control, a stimulus is an external input or disturbance that can influence the behavior of the system. This stimulus can lead to changes in the system's output, and the control system aims to minimize the effects of such disturbances through feedback and control actions.

The other options are related terms but do not specifically refer to externally introduced signals:
Option A: feedback refers to the process of taking a portion of the output and feeding it back to the input of a system to regulate its behavior.
Option C: signal is a general term for a function representing information or data, which can include control signals, stimuli, or any other data-carrying function.
Option D: gain control is related to adjusting the amplification or gain of a signal within a control system but is not the term used to describe externally introduced signals.
So, the correct answer is Option B: stimulus.