The spring will exert maximum force when the ball is at its lowest position. If the ball has descended through a distance $\mathrm{x}$ to reach the position.

$\mathrm{mgx}=\left(\frac{1}{2}\right){\mathrm{Kx}}^2\mathrm{ }\mathrm{or}\mathrm{ }\mathrm{x}=\frac{2\mathrm{mg}}{\mathrm{k}}$.....(i)

For block $\mathrm{B}$ to leave contact spring force, $\mathrm{Kx}=\mathrm{Mg}$.....(i)

Comparing equations (i) and (ii), $\mathrm{m}=\frac{\mathrm{M}}{2}$