Potentiometer null test for cell e.m.f. Steps:

• Battery drives current through series R1 and R2, creating potential difference V_{R1} = E \times R1/(R1 + R2) across R1, set to 1.5 V if E = 6.0 V.
• Test cell connects across X-Y (parallel to R1) with galvanometer in series, opposing V_{R1}.
• Zero galvanometer deflection occurs when cell e.m.f. equals V_{R1} with matching polarity, confirming no branch current.
• Galvanometer detects balance qualitatively via null point.

Why D is correct:

• Galvanometer acts as null detector in potentiometer method (Kirchhoff's law: zero current when voltages balance), needing only zero-deflection indication, not ampere calibration.

Why the others are wrong:

• A: Zero reading means cell e.m.f. equals V_{R1}, which is 1.5 V only if battery e.m.f. is 6.0 V; otherwise indicates different value.
• B: Battery requires 6.0 V e.m.f. to produce 1.5 V across R1 with fixed R1, R2 for the test.
• C: Reverse connection adds e.m.f. to V_{R1}, causing deflection; correct opposition polarity needed for balance.

Final answer: D