A potentiometer is used to measure the emf of a cell. At null point, no current flows through _.

Explanation

At the null point of a potentiometer, the potential difference across the potentiometer wire is equal to the emf of the cell being measured. This results in zero potential difference across the galvanometer, meaning no current flows through the cell circuit. However, the main circuit, including the potentiometer wire, continues to carry current to maintain the potential difference across the potentiometer wire. Therefore, the correct answer is that no current flows through the cell circuit, while the main circuit remains active to facilitate the measurement. Option B is correct, as it accurately describes the condition at the null point. Options A, C, and D are incorrect because they suggest that no current flows through the main circuit or potentiometer wire, which is necessary for the potentiometer's operation.