The octahedral complex of a metal ion ${\mathrm{M}}^{3+}$ with four monodentate ligands ${\mathrm{L}}_1,{\mathrm{L}}_2,{\mathrm{L}}_3$ and ${\mathrm{L}}_4$ absorb wavelengths in the region of red, green, yellow and blue, respectively. The increasing order of ligand strength of the four ligands is

The larger the ligand strength, larger the splitting of d orbitals. Hence, d-d transition has larger energy or lesser wavelength. Since the energy of absorbed light follows the order

Blue > Green > Yellow > Red

Thus, the ligand strength will follow the order

${\mathrm{L}}_4>{\mathrm{L}}_2>{\mathrm{L}}_3>{\mathrm{L}}_1$ i.e. ${\mathrm{L}}_1<{\mathrm{L}}_3<{\mathrm{L}}_2<{\mathrm{L}}_4$ (Choice c)