Q. Assertion :A galvanometer can be used as a voltmeter to measure the voltage across a given section of the circuit
Reason : For this it must be connected in parallel with that section of the circuit

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Solution:

To convert a galvanometer into a voltmeter, a high-value resistance is to be connected in series with it.
And, once the galvanometer converted into a voltmeter, it should be connected in the circuit as the voltmeter used to be connected, in parallel with that section of the circuit.
So, Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.