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Paramjeet verma Best Answer
The acidity of Aqua complexes of transition metals depends on charge to size ratio. Higher the charge to size ratio, more is the electron pull towards the central atom and more easily the the H+ ion are released and thus higher the acidity. Accordingly, Fe3+ has the highest charge to size ratio followed by Fe2+ , Mn2+ and Ca2+. Thus the correct order of acidity should be: Fe3+ > Fe2+ > Mn2+ > Ca2+ . Correct order of pKa : Ca2+> Mn2+ > Fe2+> Fe3+. option (b) is correct