Ambidentate ligands are ligands that can coordinate to a metal ion through different atoms. In other words, they have multiple sites on the molecule where they can form a coordinate bond with a metal ion. Among the options provided, the nitro (-NO2) ligand is an example of an ambidentate ligand.

Nitro ligand can coordinate to a metal ion through two different atoms: nitrogen (N) or oxygen (O). The nitrogen atom can donate its lone pair of electrons to the metal ion, forming a coordinate bond. Similarly, the oxygen atom can also donate its lone pair of electrons to the metal ion, forming a coordinate bond.

Explanation of each option:

a) Oxalate: Oxalate is a bidentate ligand, meaning it can form two coordinate bonds with a metal ion through its two oxygen atoms.

b) Carbonyl: Carbonyl ligands coordinate to a metal ion through the carbon atom and its double bond with oxygen. It is not an ambidentate ligand because it can only form a coordinate bond through one atom.

c) Nitro: Nitro ligands can coordinate to a metal ion through either the nitrogen atom or the oxygen atom. It is an ambidentate ligand because it has multiple sites of coordination.

d) Acetate: Acetate is a bidentate ligand, similar to oxalate. It can form two coordinate bonds with a metal ion through its two oxygen atoms.

In summary, nitro (-NO2) is the correct answer because it is an example of an ambidentate ligand, capable of forming coordinate bonds with a metal ion through either the nitrogen or oxygen atom.