So if there are strong field ligands in complex, pairing of electrons will take place, due to which number of unpaired electrons decrease due to which magnetic moment decrease.
$\left(\mathrm{c}\right)$ Splitting power of ethylenediamine (en) is greater than fluoride $\left({\text{F}}^{-}\right)$ ligand therefore more energy absorbed by ${\left[\text{Co}\left(\text{en}\right)\right]}^{3+}$ as compared to  ${\left[{\text{CoF}}_{\text{6}}\right]}^{3-}$

So wave length of light absorbed by ${\left[\text{Co}\left(\text{en}\right)\right]}^{3+}$ is lower than that of ${\left[{\text{CoF}}_{\text{6}}\right]}^{3-}$

$\text{(d) }{\mathrm{\Delta }}_{\mathrm{t}}=\frac{4}{9}{\mathrm{\Delta }}_0$

$\text{so if }{\mathrm{\Delta }}_0=18,000 {\text{cm}}^{-1}$

${\mathrm{\Delta }}_{\text{t}}=\frac{4}{9}\times 18000=8000 {\text{cm}}^{-1}$

Statement $\left(\mathrm{a}\right)$ and $\left(\mathrm{d}\right)$ are incorrect.