Potential divider null method for emf testing Steps:

• Battery drives current through series R1 and R2, creating PD across R1 of E × R1/(R1 + R2).
• Connect cell (emf ε) and galvanometer in series across X-Y (parallel to R1), opposing the PD.
• Zero galvanometer deflection occurs when ε equals PD across R1, by Kirchhoff's loop rule (net voltage zero in cell-galvanometer-R1 loop).
• Repeat across R2 if its PD is set to 1.5 V for equivalent null test.

Why D is correct:

• Kirchhoff's voltage law ensures null deflection when cell emf matches PD across R2 in the cell-galvanometer-R2 loop, allowing symmetric testing.

Why the others are wrong:

• A: Non-zero deflection means ε ≠ 1.5 V, opposite of the statement.
• B: Diagram implies 6.0 V is required to produce 1.5 V PD across the chosen resistor with fixed R1, R2.
• C: Galvanometer must detect current deflection, requiring ampere calibration for sensitivity.

Final answer: D