Explanation: An op-amp circuit is operated open-loop when there is no negative feedback path from the output back to the input. Without negative feedback, the op-amp's extremely high intrinsic gain causes the output to quickly saturate at one of the power supply rails (a logic high or low state) based on the small voltage difference between its inputs. A **comparator (D)** is designed to operate precisely in this open-loop configuration. It takes two input voltages and outputs a digital high or low signal, indicating which input is greater. This saturating behavior is its intended function. Conversely, non-inverting amplifiers (A), inverting amplifiers (B), and active filters (C) all use negative feedback. Negative feedback is crucial for these circuits to control their gain, stabilize their operation, and ensure the op-amp remains in its linear operating region, allowing them to amplify or filter analog signals accurately.