A parallel plate capacitor is made of two plates of length l , width w and separated by distance d . A dielectric slab (dielectric constant K) that fits exactly between the plates is held near the edge of the plates .It is pulled into the capacitor by a force $\;F=-\large\frac{\partial U}{\partial x}\;$ where U is the energy of the capacitor up to distance x (See figure) .If the charge on the capacitor is Q then the force on the dielectric when it is near the edge is :

Question

$(a)\;\large\frac{Q^{2}d}{2wl^{2} \in_{0}}\; \normalsize K\qquad(b)\;\large\frac{Q^{2}w}{2dl^{2} \in_{0}}\; \normalsize (K-1)\qquad(c)\;\large\frac{Q^{2}d}{2wl^{2} \in_{0}}\; \normalsize (K-1)\qquad(d)\;\large\frac{Q^{2}w}{2dl^{2} \in_{0}}\; \normalsize K$