(c) Loss of kinetic energy $= \frac{1}{2}\frac{m_{1}m_{2}}{m_{1}{+ m}_{2}}\left( v_{1} - v_{2} \right)^{2}$
$= \frac{1}{2}\frac{M \times M}{(M + M)}\left( v_{1} - v_{2} \right)^{2}$
$= \frac{M \bullet M}{2(2M)}\left( v_{1} - v_{2} \right)^{2}$
$= \frac{M}{4}\left( v_{1} - v_{2} \right)^{2}$