The sensitivity of a potentiometer can be increased by
(A) Increasing the series resistance in the primary circuit
(B) Decreasing the length of the potentiometer wire
(C) Using a thin and high resistivity wire as a potentiometer wire
(D) Increasing the length of the wire

Hint: A potentiometer may be a three-terminal resistor with a sliding or rotating contact that forms an adjustable potential divider which is used as a controller dial in various machines. If only two terminals of the potentiometer are used, that is, one end of the potentiometer and the wiper, it acts as a variable resistor or a rheostat. Keeping this in mind we can answer the given question.

Complete step-by step answer:
The sensitivity of a potentiometer is inversely proportional to its potential gradient.
With an increase in the series resistance of the wire, the potential gradient of the wire decreases which in turn helps us to measure the emf more accurately.
Also Increase in the length of the wire decreases and the potential gradient of the wire helps us to measure the emf more accurately.

Hence, the correct answer is Option D.

Note: Usually the two ends of a potentiometer acts as the two extremes of a range of resistances and the middle pin acts as a pin to vary the overall resistance provided by the potentiometer. It is a mechanical device capable of alternating the flow of current through it.