$A$ block of mass $m$ is pushed against a spring with spring constant $k$ which is attached to a wall. The block slides on a frictionless table as shown in the figure. The natural length of the spring is $\ell_0$ and it is compressed to half of its natural length when the block is released. What will be the final velocity of the block?

• A
  $\frac{{\ell _0}}{2}\sqrt {\frac{k}{m}} $
• B
  $\frac{{\ell _0}}{4}\sqrt {\frac{k}{m}} $
• C
  $\frac{1}{2}\sqrt {\frac{{k{\ell _0}}}{m}} $
• D
  $\sqrt {\frac{{k{\ell _0}}}{{2m}}} $