Identify weakest acid from following.

The given acids are oxoacids of chlorine.

The chemical formulas and oxidation states of chlorine in these acids are:
Hypochlorous acid: $HClO$ (Oxidation state of $Cl = +1$)
Chlorous acid: $HClO_2$ (Oxidation state of $Cl = +3$)
Chloric acid: $HClO_3$ (Oxidation state of $Cl = +5$)
Perchloric acid: $HClO_4$ (Oxidation state of $Cl = +7$)

The acidic strength of oxoacids of the same halogen increases with an increase in the oxidation number of the halogen atom. This is because a higher oxidation state of the central atom leads to greater polarization of the $O-H$ bond, making it easier to release an $H^+$ ion. Alternatively, the stability of the conjugate base increases with the number of oxygen atoms due to greater resonance stabilization.

Thus, the order of acidic strength is $HClO < HClO_2 < HClO_3 < HClO_4$.

Hypochlorous acid is the weakest acid among the given options.