A solid sphere of mass M, radius R and having moment of inertia about an axis passing through the centre of mass as I, is recast into a disc of thickness t, whose moment of inertia about axis passing through its edge and perpendicular to the plane remains I. Then, radius of the disc will be

Question

$$\begin {array} {1 1} (a)\;\frac{R}{\sqrt {15}} & \quad (b)\;R \sqrt {\frac{2}{15}} \\ (c)\;\frac{2R}{\sqrt {15}} & \quad  (d)\;\frac{R}{4} \end {array}$$