Four thin rods of same mass M and same length l, form a square as shown in the figure. Moment of inertia of this system about an axis through centre O and perpendicular to its plane is

Moment of inertia of rod AB about point P=112Ml2M.I. of rod AB about point O=Ml212+Ml22=13Ml2[by the theorem of parallel axis]and the system consists of 4 rods of similar type so by the symmetry ISystem =43Ml2.