The exothermic hydration enthalpies of the given trivalent cations are: Sc+3=3960kJ∕mole Fe+3=4429kJ∕ mole Co+3=4653kJ∕mole Cr+3=4563kJ∕mole Hence Sc+3 is least hydrated; so least stable (not most stable) Fe+2 contains 4 unpaired electrons where as Mn+2 contains 5 unpaired electrons hence (4) is incorrect.