Momentum would be maximum when KE would be maximum and this is the case when total elastic PE is converted to KE.
According to conservation of energy
$\frac{1}{2}\,k\,L^{2}-\frac{1}{2}\,Mv^{2}$
$k\,L^{2}=\frac{\left(Mv\right)^{2}}{M}$
$MkL^{2}=p^{2}\,\left(p=Mv\right)$
$\therefore p=L\sqrt{Mk}$