The correct set up for studying the dependence of the current on the potential difference across a resistor is:
C
In circuit C, voltmeter is connected across the resistor in parallel and ammeter is connected in series with the voltage supply. Therefore current will be measured through the ammeter and voltage will be measured through the voltmeter across the resistor. Now, as we vary the voltage, the current through the circuit will vary which will be measured by the ammeter.