a
\(1.\) \(\left[{Fe}\left({H}_{2} {O}\right)_{6}\right]^{3+}\)

\({Fe}^{3+}:[{Ar}] 3 {~d}^{5}\)

Hybridisation : \({sp}^{3} {~d}^{2}\)

Magnetic nature : Paramagnetic (so this complex response to external magnetic field)

\(2.\) \(\left[{Ni}({CN})_{4}\right]^{2-}\)

\({Ni}^{2+}:[{Ar}] 3 {~d}^{8}\)

Hybridisation: \({ds} {p}^{2}\)

Magnetic nature : diamagnetic

\(3.\) \(\left[{Co}({CN})_{6}\right]^{3-}\)

\({Co}^{3+}:[{Ar}] 3 d^{6}\)

Hybridisation : \({d}^{2} {~s} {p}^{3}\)

Magnetic nature: diamagnetic

\(4.\) \(\left[{Ni}({CO})_{4}\right]\)

\({Ni}:[{Ar}] 3 {~d}^{8} 4 {~s}^{2}\)

Hybridisation : \({sp}^{3}\)

Magnetic nature : diamagnetic