The acidic strength of oxoacids of the same halogen increases with an increase in the oxidation state of the central halogen atom. The oxidation states of chlorine in the given acids are: +1 in $HOCl$ (hypochlorous acid), +3 in $HOClO$ (chlorous acid), +5 in $HOClO_2$ (chloric acid), and +7 in $HOClO_3$ (perchloric acid). As the oxidation state of the central chlorine atom increases, its electron-withdrawing tendency increases, which strongly polarises and weakens the O-H bond, facilitating the easier release of an $H^+$ ion. Furthermore, the conjugate base formed after losing an $H^+$ is more resonance-stabilized when more oxygen atoms are present to disperse the negative charge. Therefore, the correct increasing order of acid strength is $HOCl < HOClO < HOClO_2 < HOClO_3$.