The oxoanion which contains all equivalent M-O bond is :
(I) ${\mathrm{CrO}}_4^{2-}$ (II) ${\mathrm{MnO}}_4^{-}$ (III) ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$

In ${\mathrm{CrO}}_4^{2-}$, all Cr-O bonds are equal and has the same bond energy. Similarly, bond strength of all Mn-O bonds in ${\mathrm{MnO}}_4^{-}$.is same because all Mn-O bonds are equal. But in ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-},$ there are two Cr-O bonds of different bond strength because the bond length of one Cr-O is 179 pm and other is 163 pm.