Of the given anions, the strongest Bronsted base is

The correct option is A $Cl{O}^{-}$
(i) Lower the oxidation state of central atom, weaker will be oxy acid.
(ii) Weaker the acid, stronger will be its conjugate base.
Oxidation state of Cl in HClO is +1 in $HCl{O}_2$ is +3, in $HCl{O}_3$ is +5, and in $HCl{O}_4$ is +7
The central atom Cl has the least oxidation state in HClO. Hence, HClO will be the weakest acid and the corresponding conjugate base of $Cl{O}^{-}$ is the strongest Bronsted base.