b
 Let final velocity of the block of    
  mass \( 4 m = v'\)   
  Initial velocity of block of mass \( 4 m = 0 \)   
  Final velocity of block of mass \( m = 0\)   
  According to law of conservation of   
  linear momentum

\(\begin{gathered}
  mv + 4m \times 0 = 4mv' + 0 \Rightarrow v' = v/4 \hfill \\
  Coefficient\,of\,restitution, \hfill \\
  e = \frac{{\operatorname{Re} letive\,velocity\,of\,separation}}{{\operatorname{Re} letive\,velocity\,of\,separation}} \hfill \\
  \,\,\, = \,\frac{{v/4}}{v} = 0.25 \hfill \\ 
\end{gathered} \)