Explanation

(b) Moment of inertia the rotational inertia of an object.
Moment of inertia of circular ring about an axis passing through it centre and perpendicular to its plane is
$I_{ring} = MR^{2}$
Similarly, for a circular disc about the same axis of rotation
$I_{disc} = \frac{1}{2}MR^{2}$
Hence, $\frac{I_{ring}}{I_{disc}} = \frac{MR^{2}}{\frac{1}{2}MR^{2}} = \frac{2}{1}$