The effective atomic number of $\mathrm{Co}{\left(\mathrm{CO}\right)}_{4 }$is 35 and hence is less stable. It attains stability by :

The EAN number is 35 which does not equal 36.

Thus, one electron should be added to achieve Sidgwick's rule of EAN. Therefore, it becomes stable by dimerization or Co reduction (after that EAN value become 36).

EAN value: $\mathrm{Co}(\mathrm{CO}{)}_4,27+8=35$

$\mathrm{Co}(\mathrm{CO}{)}_4+e^{-}⟶{\left[\mathrm{Co}(\mathrm{CO}{)}_4\right]}^{-}\mathrm{EAN}=36$ or 

$2\mathrm{Co}(\mathrm{CO}{)}_4\to \left[{\mathrm{Co}}_2(\mathrm{CO}{)}_8\right]\mathrm{EAN}=36$

Hence correct option is (b, c).