Arrange the following metal complex/compounds in the increasing order of spin only magnetic moment. Presume all the three, high spin system. (Atomic numbers Ce = 58, Gd = 64 and Eu = 63)

a) (NH₄)₂[Ce(NO₃)₆]
b) Gd(NO₃)₃
c) Eu(NO₃)₃

a) (NH₄)₂[Ce(NO₃)₆]=$\left[\mathrm{Xe}\right]4{\mathrm{f}}^{2}6{\mathrm{s}}^2$ , Ce has +4 oxidation state  $\mathrm{\mu }=0$

b)$\mathrm{Gd}{\left({\mathrm{NO}}_3\right)}_3$=$\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$Gd has +3 oxidation state,7 unpaired electrons;$\mu =7.9$  

c)$\mathrm{Eu}{\left({\mathrm{NO}}_3\right)}_3$  =$\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}6{\mathrm{s}}^2$ Eu has +3 oxidation state,6 unpaired electrons; $\mu =6.9$

Hence, correct option is: (A) a < c < b