c
$i_1=\frac{2\,V}{5\,\Omega}=\frac{2}{5}\,A$
$i=\frac{1 V}{2 \Omega}=\frac{1}{2}\,A$
$\therefore \text { Current through voltmeter }=i-i_1$
$=\frac{1}{2}-\frac{2}{5}=\frac{5-4}{10}=\frac{1}{10}\,A$
$\therefore \text { For voltmeter }$
$2=\left(\frac{1}{10}\right) R \Rightarrow R=20\,\Omega$
