d
Suppose $V_{0}$ is the potential at Point $\mathrm{O}$ Apply Nodel Method
$\frac{\mathrm{V}_{0}-\mathrm{V}_{1}}{\mathrm{R}_{1}}+\frac{\mathrm{V}_{0}-\mathrm{V}_{2}}{\mathrm{R}_{2}}+\frac{\mathrm{V}_{0}-\mathrm{V}_{3}}{\mathrm{R}_{3}}=0$
$\frac{\mathrm{V}_{0}-20}{20}+\frac{\mathrm{V}_{0}-30}{30}+\frac{\mathrm{V}_{0}-60}{60}=0$
$6 V_{0}=180$
$V_{0}=30 \,V$
$\mathrm{i}_{2}=\frac{\mathrm{V}_{0}-\mathrm{V}_{2}}{\mathrm{R}_{2}}=\frac{30-30}{\mathrm{R}_{2}}=0$
