B

In the potentiometer experiment, the aim is to measure the potential difference across a component without drawing any current from it. When a balance is achieved on the potentiometer, the potential difference across the galvanometer is equal to the potential difference across the segment of the wire where the balance is obtained. At this point, the potential difference is zero across the galvanometer, meaning no current flows through it. Thus, the correct answer is Option B: Galvanometer circuit.