The correct statement regarding $[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3{]}^{3+}$ complex is

$[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3{]}^{3+}$is  inner orbital complex and diamagnetic
due to strong bidentate ligand, go for pairing and forms inner octahedral orbital complex (${\mathrm{d}}^2{\mathrm{Sp}}^3$). As it does not have unpaired electrons, therefore it is diamagnetic in nature.