The order of energy absorbed, which is responsible for the color of complexes, is: (A) $[Ni(H_2O)_2(en)_2]^{2+}$ (B) $[Ni(H_2O)_4(en)]^{2+}$ (C) $[Ni(en)_3]^{2+}$

The energy of light absorbed by a coordination complex is directly proportional to its crystal field splitting energy ($\Delta_o$), which in turn depends on the field strength of the ligands present. According to the spectrochemical series, ethylenediamine ($en$) is a stronger field ligand than water ($H_2O$). As the number of $en$ ligands replacing $H_2O$ increases, the magnitude of $\Delta_o$ increases, leading to the absorption of light of higher energy (shorter wavelength). The number of $en$ ligands in the given complexes is: (C) $[Ni(en)_3]^{2+}$ : 3 $en$ ligands (Maximum $\Delta_o$) (A) $[Ni(H_2O)_2(en)_2]^{2+}$ : 2 $en$ ligands (B) $[Ni(H_2O)_4(en)]^{2+}$ : 1 $en$ ligand (Minimum $\Delta_o$) Therefore, the correct order of energy absorbed is C > A > B.