The Paramagnetism in Complex Ions of Transition Metals Such as CoF₆³

The paramagnetism in complex ions of transition metals such as CoF₆^{3 -} or NiCl₄^{2 -} is a result of:

A) the charge on the ions.
B) electron pair donation by the ligands.
C) unpaired d-orbital electrons on the metal ions.
D) the square planar geometry possible with these metals.
E) paired d-orbital electrons on the metal ions.