Chapter 4 - Sections 4.1-4.9 - Exercises - Problems by Topic - Page 189: 94c

$+5$

O normally has an oxidation state of $-2$ and the molecule $ClO_{3}^{-}$ has a $-1$ charge so the sum of the oxidation states of all atoms must be $-1$. This means the oxidation state of Cl must be $+5$ as $3(-2)+(+5)=-1$