Explanation

(c) Voltmeter is an instrument which measure the potential difference between two points. A high resistance is connected in series with coil of the galvanometer to convert it into voltmeter. This resistance is either 2000$\Omega$ or more than that.
Resistance connected in series is given by
$R = \frac{V}{I_{g}} - G\ \ \ \ \ \ ..(i)$
Eq. (i) is the value of the resistance required to convert the galvanometer to voltmeter of range 0 to V.
From the relation
$R = \frac{V}{I_{g}} - G$
$R = \frac{nV}{\left( \frac{V}{G} \right)} - G$
$or\ \ \ \ R = nG - G = G(n - 1)$
Hence, $R = (n - 1)\ G$