JEE Main 2025 Apr 3 Shift 1, Chemistry Q14: Colour, Magnetism And Stability Of Co-ordination Compounds

Cobalt is in the $+3$ state in every complex, so the splitting depends on the ligand field strength alone. From the spectrochemical series the strength order is

${\text{CN}}^{-}>{\text{NH}}_3>{\text{H}}_2\text{O}>{\text{Cl}}^{-}$.

Replacing weaker-field ligands by stronger-field ones increases ${\Delta }_o$, and the energy of the absorbed photon $E=\frac{hc}{\lambda }$. Hence stronger field $\Rightarrow$ larger ${\Delta }_o$ $\Rightarrow$ shorter $\lambda$ absorbed.

Ranking ${\Delta }_o$: $C>B>A>D$ (six ${\text{CN}}^{-}$ > six ${\text{NH}}_3$ > five ${\text{NH}}_3$ + ${\text{H}}_2\text{O}$ > five ${\text{NH}}_3$ + ${\text{Cl}}^{-}$).

So in $\lambda$ (reverse order): $D>A>B>C$.