I know that the OH^- ions from water are enough for the Al(OH)3 and Fe(OH)3 precipitates to appear and we've been taught that K,Na compounds are soluble in water too.
And Mg(OH)2 needs a higher OH^- concentration to precipitate.
I don't know what you mean by "strongest cation" especially when you follow it with Al(OH)3 and Fe(OH)3. If you're going down the OH^- path, I would pick K because KOH would be the strongest base but I may not have guessed right about where you're going with this question. Let me know if I've guessed wrong.
