The Murray loop test locates a ground (earth) fault in an underground cable by turning the cable itself into part of a measuring bridge, so its whole accuracy rests on the principle of the Wheatstone bridge and the balance of four resistance arms.

The faulty core and a healthy core are joined together at the far end of the cable to form a continuous loop. This loop supplies two of the four bridge arms, one running up to the fault and one running back.

The remaining two arms must be supplied by a set of known, adjustable resistances, that is a resistance bridge circuit with a ratio arm or calibrated resistance box.

The operator varies these known resistances until the galvanometer shows a null (no deflection). At balance the ratio of the known arms equals the ratio of the cable arms.

From that balance ratio and the known total loop length, the exact distance to the fault is calculated. Without a defined, adjustable known resistance there is no reference to balance against and no null can be found.

A digital voltmeter only reads potential difference and cannot form ratio arms; a clamp meter or tong tester reads load current on a live conductor, not a bridge balance; an earth-leakage relay only trips on residual current and measures no distance.

Hence, the answer is A known resistance bridge circuit.