The plates in a parallel plate capacitor are seperated by a distance 'd' with air as a medium between the plates. In order to increase the capacity by 66%, a dielectric slab of dielectric constant '5' is introduced between the plates. What is the thickness of the dielectric slab ?

Question

$\begin {array} {1 1} (1)\;\large\frac{d}{4} & \quad (2)\;\large\frac{d}{2} \\ (3)\;\large\frac{5d}{8} & \quad (4)\;d \end {array}$