Which one of the following is an ambidentate ligand ?

An ambidentate ligand is a unidentate ligand that has more than one donor atom but coordinates to the central metal atom or ion through only one donor atom at a time.

Thiocyanate ion ($SCN^{-}$ or $NCS^{-}$) has two donor atoms, sulfur ($S$) and nitrogen ($N$). It can bind to the metal through either the sulfur atom (thiocyanato) or the nitrogen atom (isothiocyanato). Thus, it is an ambidentate ligand.

Ethane-$1,2$-diamine is a bidentate ligand.

Ethylenediaminetetraacetate ion is a hexadentate ligand.

Oxalate is a bidentate ligand.

Answer: Thiocyanate