$\mathrm{Cl}-\mathrm{O}$ bond length is 170 pm in the oxyanion

As the number resonating structure increases then such molecule cannot have fixed bond lengths as that of single, double and triple bonds. The structures of hypochlorite, chlorite, chlorate and perchlorate are

${\mathrm{ClO}}^{-}$, $\mathrm{Cl}{{\mathrm{O}}_2}^{-}$, $\mathrm{Cl}{{\mathrm{O}}_3}^{-}$ and $\mathrm{Cl}{{\mathrm{O}}_4}^{-}$

Except ${\mathrm{ClO}}^{-}$ all other anions participate in resonance and have resonating Structures and have less bond lengths compared to ${\mathrm{ClO}}^{-}$   

Hence the correct option is (A).