Instrument used to measure electric potential difference is
A. Generator
B. Voltmeter
C. Ammeter
D. None

Hint: For the instrument to measure the potential difference between the two points, it must be connected across it. Do give accurate reading; the current passing through it must be zero.

Complete step by step answer:
An instrument used to measure the electric potential difference between the two points in an electric circuit is voltmeter. We know that the voltmeter is always connected parallel to the electric component in the circuit. For the instrument to accurately measure the electric potential difference between the two points, the current passing through it must be zero.
We can show the circuit connection of voltmeter and ammeter in the electrical circuit as shown in the figure below,

We know that the resistance of an ideal voltmeter is zero. According to Ohm’s law, , where, I is the current, V is the voltage and R is the resistance. We see the current is inversely proportional to the resistance. Therefore, the current flowing through the voltmeter is almost zero due to its extremely high resistance. Therefore, all of the current in the circuit passes through the electric component across which we measure the electric potential difference.
The generator is not an instrument which measures both potential difference and current in the circuit. It is the instrument which generates electric current from the mechanical work.
Therefore, the option (A) is incorrect.
We know that ammeter is the instrument which measures electric current in the circuit. It is always connected in the series with electric components. Therefore, the option (C) is incorrect.

So, the correct answer is option (B).

Note:
Students should know that only the ideal voltmeter has infinite resistance. The voltmeters we use has extremely high resistance but not infinite. Therefore, not much but little amount of current also passes through the voltmeter. But this does not affect the measurements up to 0 to 300 V and provide us accurate measurement with accuracy of 1% full scale reading.