For measurement of potential difference, potentiometer is preferred in comparison to voltmeter because
A. Potentiometer is more sensitive than voltmeter.
B. The resistance of the potentiometer is less than voltmeter.
C. potentiometer is cheaper than voltmeter.
D. potentiometer does not take current from the circuit.

Hint: We should know that while measuring the potential difference at a certain point, the voltage in the circuit must not be altered because of the connecting instrument. This will lead to variation in the readings taken and actual potential difference. A voltmeter doesn’t have infinite resistance and a potentiometer doesn’t require passage of current through the galvanometer for measurement.

Complete step-by-step answer:
Firstly we must know the characteristics of a potentiometer. The potentiometer works on a comparative method rather than the deflection method for determining the unknown voltages which is very accurate. It measures null points that don't require the power for measurement. As no current flows through it when it is balanced, their working is free from the source resistance.
Now, if we look at the working of a voltmeter, a small amount of current passes through the voltmeter when we connect it to a circuit, there will be a voltage drop across it. So it will not be reading the exact potential difference.
So, we can conclude that when we use a voltmeter, it doesn’t have infinite resistance and hence it allows passage of finite current through it. So, the voltage across the resistor changes a little bit. But when we use a potentiometer for the same measurement, it doesn’t require current to pass through the galvanometer. Thus no change in voltage will occur.

So, the correct answer is “Option D”.

Note: It is very important to take accurate measurements while taking a reading. So we prefer a potentiometer over voltmeter for better results. We can also use an ideal voltmeter which has an infinite internal resistance. So it will not be passing current through the voltmeter. But there is no such voltmeter as ideal because infinite resistance is only a hypothetical concept. But, we can improve the accuracy of a voltmeter reading by using a voltmeter of high resistance.