The order of energy absorbed which is responsible for the color of complexes:(A) $[\text{Ni}({\text{H}}_2\text{O}{)}_2(\text{en}{)}_2{]}^{2+}$ (B) $[\text{Ni}({\text{H}}_2\text{O}{)}_4(\text{en}){]}^{2+}$ (C) $[\text{Ni}(\text{en}{)}_3{]}^{2+}$

The energy absorbed ($E$) is proportional to the crystal field splitting (${\Delta }_o$). ${\Delta }_o$ depends on the field strength of the ligands.Ethylenediamine ($\text{en}$) is a stronger ligand than water (${\text{H}}_2\text{O}$).(C) has $3$ 'en' ligands (Strongest field).(A) has $2$ 'en' ligands.(B) has $1$ 'en' ligand (Weakest field).Order of field strength and thus absorbed energy: $C>A>B$.