d
For a parallel plate capacitor: $\quad C =\frac{ A \varepsilon_{0}}{ d }$
Volume of capacitor $= Ad$
Energy stored in a capacitor $= E =\frac{1}{2} CV ^{2}$
Energy stored per unit volume $= E ^{\prime}=\frac{\frac{1}{2} CV ^{2}}{ Ad }$
$E ^{\prime}=\frac{1}{2} \frac{\frac{A \varepsilon_{0}}{ d } V ^{2}}{ Ad }=\frac{1}{2} \varepsilon_{0} \frac{ V ^{2}}{ d ^{2}}$