"Correct option would be b)
Metallic carbonyls involves both sigma and pi-bonding. pi-bonding involves the back donation of metal d-electrons into vacant orbital of CO. Larger the electrons involve in back-bonding, lesser will be the bond order.
CO is strong field ligand. The electronic configuration of metal ion in presence of CO will be as follows:
a) Mn+ = 3d5, 4s1 = 3d6 (in presence of CO)
b)Fe0 = 3d6, 4s2 = 3d8 (in presence of CO)
c) Cr0 = 3d5, 4s1 = 3d6 (in presence of CO)
d) V- = 3d4, 4s2 = 3d6 (in presence of CO)
In Fe(CO)5, 4 lone pairs of electrons are available on metals for back donation hence lowest will be bond order."