b
Ligand field strength \(\propto\) Energy of light absorbed \(\propto \frac{1}{\text { Wavelength of light absorbed }}\)

\(\begin{array}{*{20}{c}}
  \lambda &{{\lambda _1}}&{{\lambda _2}}&{{\lambda _3}}&{{\lambda _4}} \\ 
  {{\text{Absorbed}}}&{{\text{light}}\,\,{\text{Red}}}&{{\text{Green}}}&{{\text{Yellow}}}&{{\text{Blue}}} 
\end{array}\)

Wavelength of absorbed light decreases.

\(\therefore\) Increasing order of energy of wavelengths absorbed reflect greater extent of crystal fleld splitting, hence, higher field strength ot the ligand.

\(Energy\; blue \left(L_{4}\right)>\operatorname{green}\left(L_{2}\right)>\operatorname{yellow}\left(L_{3}\right)>r e d\left(L_{1}\right)\)

\(\therefore \quad L_{4}>L_{2}>L_{3}>L_{1}\) in fleld strength of ligands.