The oxidation number of an atom is the charge it would have if electrons were transferred completely to the more electronegative atom. Chlorine is more electronegative than most other elements, and thus has a high tendency to gain electrons. The oxidation number of chlorine in KCIO3 is +5, in CI2O7 is +7, in ZnCI2 is -1, and in HCI is -1. Therefore, the only option where the oxidation number of chlorine is +1 is NaCIO.