c
Velocity after collision

\(V_{2}=\frac{\left(m_{2}-m_{1}\right) u_{2}+2 m_{1} u_{1}}{m_{1}+m_{2}}\)

\(V_{2}=\frac{(5 m-m) 0+2 m \cdot u_{0}}{m+5 m}=\frac{u_{0}}{3}\)

\(\% \Delta K E=\frac{\frac{1}{2} 5 m\left(\frac{u_{0}}{3}\right)^{2}-0}{\frac{1}{2} m u_{0}^{2}} \times 100\)

\(=\frac{5 u_{0}^{2}}{9 u_{0}^{2}} \times 100=\frac{500}{9}=55.6 \%\)