In metal carbonyls, metal ligand $\mathrm{\sigma }$ bond is formed 
by: 

It is created by $\mathrm{\pi }$-acid ligands such as CO. $\mathrm{\pi }$-acid ligands have a single pair of electrons to donate to the metal atom and an empty antibonding molecular orbital for back bonding with the metal atom's d-orbital electrons.
So, option A is correct answer.