b
As the number of unpaired electron increases, the magnetic moment increases and hence, the paramagnetic behaviour increases.

So, \(C r^{2+}(22)=3 d^{4}(4\) unpaired electrons)

\(M n^{2+}(23)=3 d^{5}(5\) unpaired electrons)

\(F e^{2+}(24)=3 d^{6}(4\) unpaired electrons)

\(N i^{2+}(26)=3 d^{8}(2\) unpaired electrons)

So, \(\left[N i\left(H_{2} O\right)_{6}\right]^{2+}\) exhibit minimum paramagnetic behaviour