Which of the following is a hexadentate ligand?

The correct answer is option 3. Ethylenediaminetetracetate ion.

A hexadentate ligand is a ligand that can donate 6 electrons to a central metal atom. Ethylenediaminetetracetate ion (EDTA) has 6 donor atoms, 2 nitrogen atoms, and 4 oxygen atoms, so it is a hexadentate ligand.

2,2-Dipyridyl, ethylenediamine, and biphenyl are all bidentate ligands. They can donate 2 electrons to a central metal atom.