$T - Mg\cos \theta = $Centripetal force
$ \Rightarrow T - Mg\cos \theta = \frac{{M{v^2}}}{L}$
Also tangential acceleration $|{a_r}| = g\sin \theta $.


($A$) The amplitude of oscillation in the first case changes by a factor of $\sqrt{\frac{M}{m+M}}$, whereas in the second case it remains unchanged
($B$) The final time period of oscillation in both the cases is same
($C$) The total energy decreases in both the cases
($D$) The instantaneous speed at $x_0$ of the combined masses decreases in both the cases