What is the angle between electric field and equipotential surface?

A slab of copper of thickness $\frac{d}{2}$ is introduced between the plates of a parallel plate capacitor which touches one of the plates of the capacitor, where $d$ is the separation between its two plates. If the capacitance of the capacitor without copper slab is $C$ and with copper slab is $C^{\text{'}}$, then $\frac{C^{\text{'}}}{C}$ is

Choose the correct relation between polarisation and electric susceptibility of dielectric material.