Concept:A potentiometer gives the most accurate comparison of e.m.f.s of two cells because it uses the null-deflection method.
Explanation:A voltmeter draws some current from the cell, so it measures terminal voltage instead of the true e.m.f.
A galvanometer only indicates or detects the direction of a small current, so it cannot compare e.m.f.s directly.
A meter rule is used for measuring length, not electrical potential.
In a potentiometer, the cells are balanced against a wire of known resistance until no current flows.
This condition gives an accurate comparison of the e.m.f.s without drawing current from the cells.
Answer:Potentiometer (Option D).