d
For \(\left[ Fe \left( NH _3\right)_6\right]^{+2}, \Delta_0 < P\), hence the pairing of electrons does not occur in \(t_{2 g}\). Therefore complex is outer orbital and its hybridisation is \(sp ^3 d ^2\).

List \(I\) (Complexes)	List \(II\) (Hybridisation)
\(A\) \(\left[ Ni ( CO )_4\right]\)	\(I\) \(sp ^3\)
\(B\) \(\left[ Cu \left( NH _3\right)_4\right]^{2+}\)	\(II\) \(dsp^2\)
\(C\) \(\left[ Fe \left( NH _3\right)_6\right]^{2+}\)	\(III\) \(sp^3d^2\)
\(D\) \(\left[ Fe \left( H _2 O \right)_6\right]^{2+}\)	\(IV\) \(d^2sp^3\)