Among the following species the one which causes the highest CFSE ${\mathrm{\Delta }}_0$, as a ligand is:

The strongest ligand CO produces the highest CFSE ( $\left.{\mathrm{\Delta }}_0\right)$ because it occupies highest position in the spectrochemical series. For the given ligands, the relative placement in spectrochemical series is

${\mathrm{F}}^{-}<{\mathrm{NH}}_3<{\mathrm{CN}}^{-}<\mathrm{CO}$