The hybridization of ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ and ${\left[\mathrm{Co}\left({\mathrm{C}}_2{\mathrm{O}}_4\right)\right]}^{3-}$ are 

Since CO in  ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ has four unpaired electrons and forms an octahedral complex it is $s{p}^3{d}^2$hybridized

While ${\mathrm{CO}}^{3+}$ in ${\left[\mathrm{Co}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}$ has electronic configuration  $\left[\mathrm{Ar}\right]3{\mathrm{d}}^6$ it is $d^{2}s{p}^3$ hybridised.

Hence correct option is C.