The correct answer is $\boxed{\frac{G}{n-1}}$.
A voltmeter is a device used to measure the voltage between two points in an electric circuit. The resistance of a voltmeter is very high, so that it does not draw much current from the circuit being measured. This is important because the current drawn by the voltmeter could affect the voltage being measured.
The range of a voltmeter is the maximum voltage that it can measure. To convert a voltmeter of range $V$ to a voltmeter of range $nV$, we need to add a resistance in series with the voltmeter. This resistance will reduce the current that flows through the voltmeter, and therefore the voltage that it measures. The value of the resistance required is given by:
$$R = \frac{V}{nV}G = \frac{G}{n-1}$$
Option A is incorrect because it would increase the range of the voltmeter, not decrease it. Option B is incorrect because it would not change the range of the voltmeter. Option C is incorrect because it would decrease the range of the voltmeter by too much. Option D is incorrect because it is the resistance of the voltmeter itself, not the resistance that needs to be added in series to change the range.