Four spheres, each of mass M and radius r are situated at the four corners of square of side R. The moment of inertia of the system about an axis perpendicular to the plane of square and passing through its center will be

MOI of sphere A about its diameter IO'=25Mr2Now MOI of sphere A about an  axis perpendicular to the plane of square and passing through its center will be IO=IO'+MR22=25Mr2+MR22    [by the theorem of parallel axis]Moment of inertia of system (i.e. four sphere)Isys=4I0=425Mr2+MR22=25M4r2+5R2