Dien is an example of

Tridentate ligand is an example of dien.

Following is the structure of Dien (Diethylenetriamine). 

${\mathrm{H}}_2\mathrm{N}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{NH}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{NH}}_2$

It is a tridentate ligand since it has three donor atoms. Donor atoms are present in every nitrogen atom.
Hexadentate ligand ${\mathrm{EDTA}}^{4-}$ is one of them. Bidentate ligand $(\mathrm{COO}{)}_2^{2-}$ is one of them. Monodentate ligands include ${\mathrm{NO}}_2^{-}$.

Hence, option C is correct.