a
(i) \(\left[F e(C N)_{6}\right]^{3-}\)

Here, oxidation state of \(F e=+3\)

Electronic configuration of \(F e\) (ground state) \(=[A r] 3 d^{6} 4 s^{2}\)

Electronic configuration of \(F e\) (excited state) \(=[A r] 3 d^{5} 4 s^{0}\)

Number of unpaired electrons \(=5\)

(ii) \(\left[F e(C N)_{6}\right]^{4-}\)

Here, oxidation state of \(F e=+2\)

Electronic configuration of \(F e\) (ground state) \(=[A r] 3 d^{6} 4 s^{2}\)

Electronic configuration of \(F e\) (excited state) \(=[A r] 3 d^{6} 4 s^{0}\)

Number of unpaired electrons \(=4\)

(iii) \(\left[\operatorname{Cr}\left(H_{2} O\right)_{6}\right]^{3+}\)

Here, oxidation state of \(C r=+3\)

Electronic configuration of \(C r^{3+}=[A r] 3 d^{3} 4 s^{0}\)

Number of unpaired electrons \(=3\)

(iv) \(\left[ Cu \left( H _{2} O \right)_{6}\right]^{2+}\)

Here, oxidation state of \(C u=+2\)

Electronic configuration of \(C u^{2+}=[A r] 3 d^{9} 4 s^{0}\)

Number of unpaired electrons \(=1\)

A number of unpaired electrons present is directly proportional to a paramagnetic character.

So, \(\left[ Fe ( CN )_{6}\right]^{3-}\) has maximum paramagnetic character.