Explanation: Closed-loop op-amp circuits utilize negative feedback to achieve stable and predictable gain. This is accomplished by connecting a portion of the output signal back to the inverting input. If the output signal increases, the feedback network drives the inverting input in a way that counteracts this increase, effectively reducing the differential input voltage and thus controlling the output. This self-regulating action stabilizes the circuit's gain and improves linearity. Connecting the feedback to the non-inverting input (B) would result in positive feedback, which typically leads to instability, oscillation, or saturation, rather than controlled gain. Options A and D are too vague or describe connections that do not implement the essential negative feedback loop for stable gain control.