Which is the strongest acid in the following ?

The strength of oxyacids can also be decided with the help of the oxidation number of central atom. Higher the oxidation number of central atom, more acidic is the oxyacid. $H_2\stackrel{+6}{S{O}_4},H\stackrel{+5}{Cl{O}_3},H\stackrel{+7}{Cl{O}_3},H\stackrel{+7}{Cl{O}_4},H_2\stackrel{+4}{S{O}_3}$ Order of acidic nature
$HCl{O}_4>H_{2}S{O}_4>HCl{O}_3>H_{2}S{O}_3$
Since, in $HCl{O}_4$, oxidation number of Cl is highest, so, $HCl{O}_4$ is the strongest acid among the given acids.