The acidic strength of oxoacids of the same halogen increases with the increase in the oxidation state of the central halogen atom. The oxidation states of chlorine in the given oxoacids are: $HClO$: +1 $HClO_2$: +3 $HClO_3$: +5 $HClO_4$: +7 As the oxidation state of the central atom increases, its ability to draw electron density towards itself increases. This polarises the O-H bond to a greater extent, making it easier to release an $H^+$ ion. Therefore, the correct order of acidity is $HClO < HClO_2 < HClO_3 < HClO_4$.