b
In series : Potential difference $\propto$ $R$
When only $S_1$ is closed ${V_1} = \frac{3}{4}E = 0.75\,E$
When only $ S_2$ is closed ${V_2} = \frac{6}{7}E = 0.86\,E$
and when both $S_1$ and $S_2$ are closed combined resistance of $6R$ and $3R$ is $2R$
${V_3} = \left( {\frac{2}{3}} \right)E = 0.67\,E$ $\Rightarrow $ ${V_2} > {V_1} > {V_3}$