The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is :

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

(a) $\left(\mathrm{NH}_{4}\right)_{2}\left[\mathrm{Ce}\left(\mathrm{NO}_{3}\right)_{6}\right]$

(b) $\mathrm{Gd}\left(\mathrm{NO}_{3}\right)_{3}$ and

(c) $\mathrm{Eu}\left(\mathrm{NO}_{3}\right)_{3}$

Answer is:

(1) $(b)<(a)<(c)$

(2) (c) $<(a)<(b)$

(3) $(a)<(b)<(c)$

(4) $(a)<(c)<(b)$