The best instrument for accurate measurement of the emf of a cell.
(A) Potentiometer
(B) Wheatstone bridge
(C) Voltmeter
(D) Ammeter

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Hint: As we know now the potentiometer is a device which we use to measure the electromagnetic force of the cell and also to compare it we use this. By using the principle of Wheatstone bridge and as we know no current flows through the galvanometer we will be able to tell the answer to this question.

Complete step-by-step solution
Potentiometer works on the principle of Wheatstone bridge i.e. no current flows through the meter. A potentiometer is a right and adaptable gadget to make electrical estimations of voltage because of the strategy includes a state of no flow move through the galvanometer, the gadget might be acclimated measure electric potential, interior opposition of a cell and think about emf of two sources.
Once a potentiometer is connected across a circuit, in zero deflection condition the potentiometer attracts no current thus the reading of the potentiometer is accurate.
Hence the option (A) Potentiometer will be correct.

Additional information
A potentiometer could be a three-terminal device utilized in a circuit as a resistor. The input voltage is applied across the electrical device. The center contact could be a wiper (sliding contact) that slides over the electrical device providing never-ending contact with the resistor providing a variable voltage between most and minimum voltage. It is usually used as volume management and tone management in amplifiers and lots of different uses.

Note
Both potentiometer & voltmeter are accustomed to measure electrical potential. However, once employing a meter there’s a free fall inside the meter, that the measurement won’t be correct. The potentiometer successively uses the null purpose (balance between two sides), the potentiometer offers correct measurement once the meter shows zero deflection.