Introduction:
In control systems, feedback refers to the process of taking a portion of the output signal and feeding it back to the input in order to regulate and correct the system's behavior. It plays a crucial role in maintaining stability and achieving desired performance in various engineering applications. In this context, the term "stimulus" refers to an external input or disturbance that affects the controlled output of the system.

Explanation:
The correct answer to the given question is option 'B', which states that any externally introduced signal affecting the controlled output is called a stimulus. Let's understand why this is the correct answer:

Feedback:
- Feedback is an important concept in control systems, where a portion of the output signal is fed back to the input.
- It allows the system to compare the actual output with the desired output and make necessary adjustments to minimize the difference between them.
- The feedback loop can be used to regulate and control various system parameters, such as gain, stability, and response time.

Stimulus:
- In the context of control systems, a stimulus refers to an external input or disturbance that affects the controlled output of the system.
- It can be any signal or event that introduces a change in the system's behavior or output.
- Stimuli can be intentional or unintentional and can arise from various sources, such as environmental conditions, user inputs, or system malfunctions.
- The purpose of feedback is to compensate for the effects of these stimuli and maintain the desired output despite their presence.

Feedback vs Stimulus:
- Feedback and stimulus are related concepts but serve different purposes in a control system.
- Feedback is the mechanism by which the system continuously adjusts its behavior based on the comparison between the desired and actual outputs.
- Stimulus, on the other hand, refers to the external inputs or disturbances that affect the output of the system.
- While feedback is used to counteract the effects of stimuli and maintain system performance, stimuli can be seen as the cause of the need for feedback.
- In other words, stimuli are the external factors that necessitate the use of feedback to achieve desired control and regulation.

Therefore, in the given question, the correct answer is option 'B' - stimulus, as it accurately represents the externally introduced signal affecting the controlled output in a control system.