The coordination number of a central metal atom in a complex is determined by :
a. the number of only anionic ligands bonded to the metal ion
b. the number of monodentate ligands around a metal ion bonded by pi-bonds
c. the number of monodentate ligands around a metal ion bonded by σ and pi-bonds both
d. the number of monodentate ligands around a metal ion bonded by σ -bonds