which of the following complex is with lowest number of unpaired electron.

${\mathrm{Ni}}^{4+}$ in ${\left[{\mathrm{NiF}}_6\right]}^{2-}$

$0-$unpaired electron

More positive charge on central atom greater will be $\mathrm{CFSE}$ so unpaired electron will be paired.

$\mathrm{B} : {\mathrm{Cu}}^{++}$ in $[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4{]}^{2+}$

$1-$ unpaired electron

$\mathrm{C} : {\mathrm{CO}}^{3+}$ in ${\left[{\mathrm{COF}}_6\right]}^{3-}$

$4-$unpaired electron

$\mathrm{D} : {\mathrm{Fe}}^{3+}$ in ${\left[\mathrm{Fe}\right(\mathrm{EDTA}\left)\right]}^{-}$

$1-$ unpaired electron