The ion that forms a white precipitate with NaOH but not with NH3 is Ag+ ion. It forms a white ppt of AgOH upon addition of aqueous solution of NaOH. Upon addition of aqueous NH3, initially it forms a similar whitish precipitate, but when added in slight excess, a soluble complex of the formula Ag(NH3)2OH is formed resulting in a clear solution. The entire process happens so quickly that one is tempted to believe that it did not form any precipitate whatsoever. Hence silver ion (Ag+) should be the correct answer for this question.