ans c:

40. A wedge of mass M fitted with a spring of stiffness k' is kept a smooth horizontal surface. A rod of mass m is kept on the wedge as shown in the figure. System is in equilibrium and at rest Assuming  that all surfaces are smooth, the potential energy stored in the spring is : 

$\left(\mathrm{A}\right) \frac{{\mathrm{mg}}^2{\tan }^2\mathrm{\theta }}{2\mathrm{K}} \left(\mathrm{B}\right) \frac{{\mathrm{m}}^2 \mathrm{g} {\tan }^2\mathrm{\theta }}{2\mathrm{K}} \left(\mathrm{C}\right) \frac{{\mathrm{m}}^2 {\mathrm{g}}^2 {\tan }^2\mathrm{\theta }}{2\mathrm{K}} \left(\mathrm{D}\right) \frac{{\mathrm{m}}^2 {\mathrm{g}}^2 {\tan }^2\mathrm{\theta }}{\mathrm{K}}$