One end of a thin uniform rod of length L and mass M1 is rivetted to the centre of a uniform circular disc of radius r and mass M2, so that both are coplanar. The distance of centre of mass of the combination from the centre of the disc is
LM12(M1+M2)
Taking origin at the centre of the disk and x-axis along the length of the rod, the mass of the rod can be considered to be concentrated at its centre (x=L2).
Now, RCM=M1×L2+M2×0M1+M2=LM12(M1+M2)
Hence, the correct choice is (b).