What is the total number of valence electrons in the chlorate ion, ${\mathrm{ClO}}_3^{-}?$

The total number of valence electrons in the chlorate ion, ${\mathrm{ClO}}_3^{-}$ is $7+6\times 3+1=26e^{-}$.

By dividing the total number of valence electrons by two, one may calculate the total number of electron pairs. ${\mathrm{ClO}}_3^{-}$ has 26 valence electrons, making a total of 13 pairs of electrons.

Hence, the option B is correct.