Regenerative feedback refers to a feedback loop in a system where the feedback signal is in phase with the input signal and adds to it, thereby reinforcing or amplifying it. This type of feedback is also known as positive feedback.

To elaborate on the concepts:

- **Positive Feedback (Regenerative Feedback)**: This occurs when the feedback signal is in phase with the input signal, causing the overall gain of the system to increase. This can lead to amplification of signals, and in electronic circuits, it can cause the system to become unstable and produce oscillations if the feedback is strong enough.

Negative Feedback: This occurs when the feedback signal is out of phase with the input signal, which tends to stabilize the system by reducing its overall gain and improving its performance in terms of linearity and bandwidth.

With these explanations, the correct answer to the question is:

 D. positive sign.

This is because regenerative feedback implies positive feedback.