Potentiometer requires precise circuit parameters for emf measurement Steps:

• Identify the standard potentiometer setup: known emf E drives current through uniform wire OR, with jockey S for null deflection via galvanometer when connected to unknown cell X.
• At balance, potential drop across length OS equals emf of X, given by ε_X = (l/L) × ε_E, where l is balancing length, L total length.
• Note labeling discrepancy: wire called OR in description but QR in options, suggesting possible figure-specific component.
• Conclude requirements from formula: uniformity and emf comparison are key, but circuit details ambiguous without figure.

Not enough information to confirm B over other options due to incomplete circuit description and labeling inconsistency.

Final answer: Not enough information.