Strongest conjugate base is

If acid is weak, its conjugate base will be strong. Greater the oxidation number of Cl, stronger the acid and thus, weaker the conjugate base.HClO+1,HClO2+3,HClO3′+5HClO4+7                                          →Oxidation number of Cl and acidic strength increases.ClO−,ClO2−,ClO3−,ClO4− ←Conjugate base strength increases.Hence, CIO- is the strongest base.