The block of mass M moving on a frictionless horizontal surface collides with a spring of spring constant k and compresses it by length L. The maximum momentum of the block after collision is 

When block of mass M collides with the spring, its kinetic energy gets converted into elastic potential energy of the spring. 

From the law of conservation of energy, 

KE = energy stored in a spring
 $\Rightarrow \frac{1}{2}M{v}^2=\frac{1}{2}k{L}^2\Rightarrow \therefore v=\sqrt{\frac{k}{M}}L$      [here M is mass, length of compression in spring = L]
Where v is the velocity of block by which it collides with spring. So its maximum momentum,
 $momentum=p=Mv=M\sqrt{\frac{k}{M}}L=\sqrt{Mk}L$
After collision, the block will rebound with same linear momentum.