The correct answer is A: Restrict both gain and Q. To prevent unwanted ringing and audio instability in an op-amp audio filter, you should restrict both gain and Q (quality factor). High gain and high Q can cause the filter to become unstable and oscillate. Q (quality factor) determines how sharp the filter response is - higher Q means a sharper, more selective filter, but also more tendency to ring and oscillate. High gain amplifies any oscillations. By restricting both gain and Q to reasonable values, you keep the filter stable while still achieving useful filtering. This is especially important in audio filters where ringing and instability would be audible and annoying. Typical audio filters use moderate Q values (1 to 5) and moderate gain to maintain stability.