b
\(\ln \left[\operatorname{Cr}\left(H_{2} O\right)_{6}\right]^{3+},\) the \(C r^{3+}\left(d^{3}\right)\) ion outer orbital complex with 3 unpaired
electrons
\(\ln \left[\operatorname{Fe}\left(H_{2} O\right)_{6}\right]^{2+},\) the \(F e^{3+}\left(d^{6}\right)\) ion outer orbital complex with 4 unpaired electrons
\(\ln \left[C u\left(H_{2} O\right)_{6}\right]^{2+},\) the \(C u^{3+}\left(d^{8}\right)\) ion contains 1 unpaired electrons
\(\ln \left[Z n\left(H_{2} O\right)_{6}\right]^{2+}\), the \(Z n^{2+}\left(d^{1} 0\right)\) outer orbital complex with o unpaired
electrons
Thus highest paramagnetism is exhibited by \(\left[F e\left(H_{2} O\right)_{6}\right]^{2+}\)