(i) There is no to and fro motion which is a must for a periodic motion to be SHM. Hence, rotation of earth about its axis is not SHM.
(it) This is a periodic motion and as it follows F = - kx (about mean position, to and fro motion) hence
(Hi) A periodic motion, oscillatory in nature about lower most point as mean position following SHM force law hence, it is SHM,
(iv) A polyatomic molecule has a number of natural frequencies. So, in general, its vibration is a superposition of SHMs of a number of different frequencies. Thus, superposition is periodic but not necessarily SHM.