In the isoelectronic series of metal carbonyl, the CO bond strength is expected to increase in the order :
a. [Mn(CO)6]+<[Cr(CO)6]<[V(CO)6]−
b. [V(CO)6]−<[Cr(CO)6]<[Mn(CO)6]+
c. [V(CO)6]−<[Mn(CO)6]+<[Cr(CO)6]
d. [Cr(CO)6]<[Mn(CO)6]+<[V(CO)6]−