a
\(C_{1}=\frac{\varepsilon_{0} K_{1} \frac{L^{2}}{2}}{\frac{d}{2}}+\frac{\varepsilon_{0} K_{3} \frac{L^{2}}{2}}{\left(\frac{d}{2}\right)}=\frac{\varepsilon_{0} L^{2}}{d}\left(K_{1}+K_{3}\right)\)

\(C_{2}=\frac{\varepsilon_{0} K_{2} \frac{L^{2}}{2}}{\frac{d}{2}}+\frac{\varepsilon_{0} K_{4} \frac{L^{2}}{2}}{\frac{d}{2}}=\frac{\varepsilon_{0} L^{2}}{d}\left(K_{2}+K_{4}\right)\)

\(\therefore \quad \frac{1}{c}=\frac{1}{c_{1}}+\frac{1}{c_{2}}\)

\(\Rightarrow \quad \frac{\mathrm{d}}{\varepsilon_{0} \mathrm{KL}^{2}}=\frac{\mathrm{d}}{\varepsilon_{0} \mathrm{L}^{2}\left(\mathrm{K}_{1}+\mathrm{K}_{3}\right)}+\frac{\mathrm{d}}{\varepsilon_{0} \mathrm{L}^{2}\left(\mathrm{K}_{2}+\mathrm{K}_{4}\right)}\)