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