Chelation is a process in which a ligand forms a complex with a metal ion by forming multiple bonds. The ligand that can form chelate complexes is the one that has multiple donor atoms capable of binding to the metal ion simultaneously.

Among the given options, the ligand that can form chelate complexes is C2O42- (oxalate ion). Here's why:

Chelating Ligands:
- A chelating ligand is capable of donating more than one pair of electrons to the metal ion, forming multiple bonds.
- The multiple bonds formed by the ligand can create a more stable and rigid complex compared to complexes formed with monodentate ligands (ligands that donate only one pair of electrons).

Explanation:
- NH3 (ammonia) is a monodentate ligand. It has one lone pair of electrons that can donate to the metal ion. Therefore, it cannot form chelate complexes.

- SCN- (thiocyanate ion) is also a monodentate ligand. It has one lone pair of electrons that can donate to the metal ion. Therefore, it cannot form chelate complexes.

- Pyridine is a monodentate ligand. It has one lone pair of electrons that can donate to the metal ion. Therefore, it cannot form chelate complexes.

- C2O42- (oxalate ion) is a bidentate ligand. It has two oxygen atoms as donor atoms, each capable of donating one pair of electrons to the metal ion. The two oxygen atoms can bind to the metal ion simultaneously, forming a chelate complex. The oxalate ion can form stable complexes with many metal ions, such as Fe2+, Mn2+, etc.

Conclusion:
Among the given options, the ligand that can form chelate complexes is C2O42- (oxalate ion). It has two donor atoms capable of binding to the metal ion simultaneously, forming multiple bonds and creating a stable complex. The other ligands (NH3, SCN-, and pyridine) are monodentate ligands and cannot form chelate complexes.