In which one of the following metal carbonyls, $\mathrm{CO}$ forms a bridge between metal atoms?

In the most common bridging mode, denoted ${\mathrm{\mu }}_2$ or simply $\mathrm{\mu }$, the $\mathrm{CO}$  ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: ${\mathrm{Co}}_2{\left(\mathrm{CO}\right)}_8$, ${\mathrm{Fe}}_2{\left(\mathrm{CO}\right)}_9$, ${\mathrm{Fe}}_3{\left(\mathrm{CO}\right)}_{12}$and ${\mathrm{Co}}_4{\left(\mathrm{CO}\right)}_{12}$. In certain higher nuclearity clusters, $\mathrm{CO}$ bridges between three or even four metals.