The capacitor plates are fixed on inclined plane p connected to a battery of emf E. A dielectric slab of mass m and dielectric constant k is inserted into the capacitor and tied to mass M using a weightless string as shown. If capacitance have plate area A, length l and distance d between them. Find M such that slab stays in equilibrium. Assume zero friction between slab and plates

Question

$(A)\;\frac{m}{2} -\frac{E^2 \in _0 A(k-1)}{2 l gd } \\ (B)\;\frac{m}{2} +\frac{E^2 \in _0 A(k-1)}{ l gd } \\ (C)\; \frac{m}{2} +\frac{E^2 \in _0 A(k-1)}{2 l gd } \\ (D)\;none$