Which of the following instrument is most accurately for comparing electromotive force (e.m.f) of two cells?

The correct answer is "Potentiometer". A potentiometer is an electrical instrument used to measure the electromotive force (e.m.f) of a cell or battery. It's the most accurate instrument for comparing the e.m.f of two cells because it measures the potential difference directly. A potentiometer works by balancing the potential difference of the cell being tested with a known potential difference from a reference cell. The e.m.f of the cell being tested can then be determined by measuring the current in the circuit and the resistance of the potentiometer. The result is a highly accurate measurement of the e.m.f of the cell.