O CO2 molecule have 2 double bonds between C and O O=C=O CO32− ion have 3 partial double bonds between C and O, due to resonance.
HCO2− ion also show resonance, but due to the presence of hydrogen atom attached to carbon atom it has less resonance hybrid, therefore, HCO2− ion have greater bond order than CO32−
Hence, the correct decreasing order of bond length is CO32−,HCO2−,CO2,CO