Manipal Entrance Test 2014 Chemistry Paper

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

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Oxidation number of Cl and acid strength increases
ClO− ClO2− ClO3− ClO4−
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Conjugate base strength increases.
Hence, CIO‌− is the strongest base.