Sensitivity of potentiometer can be increased by
a) increasing series resistance in the primary circuit
b) decreasing the length of potentiometer wire
c) using thin and high resistivity wire as potentiometer wire
d) increasing the length of the wire

The sensitivity of potentiometer is inversely proportional to the potential gradient.

With the increase in the series resistance, the potential gradient of the wire decreases thereby helps us to measure the emf more accurately.
Increase in the length of the wire decrease the potential gradient of the wire thereby helps us to measure the emf more accurately.