In which one of the following metal carbonyls, CO forms a bridge...

In the most common bridging mode, denoted ${\mu}_{2}$ or simply $\mu$, the $CO$  ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: ${Co}_{2}{\left(CO\right)}_{8}$, ${Fe}_{2}{\left(CO\right)}_{9}$, ${Fe}_{3}{\left(CO\right)}_{12}$and ${Co}_{4}{\left(CO\right)}_{12}$. In certain higher nuclearity clusters, $CO$ bridges between three or even four metals.