Which of the following ions or molecules acts as an ambidentate...

An ambidentate ligand is a monodentate ligand that contains more than one coordinating atom and can coordinate to the central metal ion through either of them.

The $\text{NO}_2^{-}$ ion has two donor atoms: nitrogen and oxygen. It can bind to the central metal ion either through the nitrogen atom (forming a nitro complex) or through an oxygen atom (forming a nitrito complex).

$\text{C}_2\text{O}_4^{2-}$ (oxalate)$and \text{NH}_2\text{CH}_2\text{CH}_2\text{NH}_2$ (ethylenediamine) are bidentate ligands.

$\text{H}_2\text{O}$ is a monodentate ligand with only oxygen as the donor atom.

Thus, $\text{NO}_2^{-}$ acts as an ambidentate ligand.

Answer: $\text{NO}_2^{-}$