A voltmeter is used:

ANS:C - in parallel with the circuit

Voltage (Electrical potential) is an electrical force applied to move a charge from one point (A) to another point (B). ie) Work done (Joules). At point A high is available and in moving the charge to point B as work is done some potential is consumed. Hence potential at B will be less than at A. Hence a potential difference between points A &B. A voltmeter is used to measure the potential difference between two points. Any measurement of difference is wrt to a reference point. Hence, a voltmeter is connected across (Parallel) two points.