d
\(\begin{array}{l}
\left( i \right)\,\,\,\,m{v_0} =  - m{v_1} + m{v_2}\\
\left( {ii} \right)\,\,{v_0} = {v_1} + {v_2}\\
\,\,\,\,\,\,\,\,\,\frac{{2m{v_0}}}{{m + M}} = {v_2}\\
K{E_f} = \frac{1}{2}mv_1^2 = \frac{1}{2}m{\left( {\frac{{M - m}}{{M + m}}} \right)^2}v_0^2 = \frac{{36}}{{100}} \times \frac{1}{2}mv_0^2\\
 \Rightarrow \,M = 4\,m.
\end{array}\)