b
\(\begin{array}{l}
According\,to\,conservation\,of\,momentum,\\
we\,get\,\\
m\,v\,\hat i + \left( {3m} \right)2v\,\hat j\, = \left( {m + 3m} \right)\bar v'\\
Where\,\bar v'\,is\,the\,final\,velocity\,after\,\\
collision\\
\bar v' = \frac{1}{4}v\hat i + \frac{6}{4}\,v\,\hat j = \frac{1}{4}v\hat i + \frac{3}{2}v\,\hat j
\end{array}\)
