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 \(= E ^{\prime}=\frac{\frac{1}{2} CV ^{2}}{ Ad }\)

\(E ^{\prime}=\frac{1}{2} \frac{\frac{A \varepsilon_{0}}{ d } V ^{2}}{ Ad }=\frac{1}{2} \varepsilon_{0} \frac{ V ^{2}}{ d ^{2}}\)