The moment of inertia of a uniform circular disc of radius R and...

We should use parallel axis theorem. Moment of inertia of disc passing through its centre of gravity and perpendicular to its plane is $I_{A B}=\frac{1}{2} M R^2$ Using theorem of parallel axes, we have $$\begin{aligned} I_{C D} & =I_{A B}+M R^2 \\ & =\frac{1}{2} M R^2+M R^2 \\ & =\frac{3}{2} M R^2\end{aligned}$$ NOTE : The role of moment of inertia in the study of rotational motion is analogous to that of mass in study of linear motion.