a
The potentiometer is almost an ideal voltmeter, hence the resistance of potentiometer is infinity.
current through potentiometer is very small, it does not draw any current from circuitwhen the potentiometer is replaced by voltmeter of finite resistance, the current through voltmeter is more than that of potentiometer. It means voltmeter draws some current from circuit.
hence $I > I 0$
Also $\mathrm{V}=I \mathrm{R}$ thus, $\mathrm{V} < \mathrm{Vo}$