d and f Block Elements Part 2

Given below are two statements : one is labelled as
"Assertion A" and the other is labelled as "Reason R".
Assertion A : In the complex Ni(CO)4 and Fe(CO)5, the metals have zero oxidation state.
Reason R : Low oxidation states are found when a complex has ligands capable of π-donor character in addition to the σ-bonding.
In the light of the above statements, choose the most appropriate answer from the option given below.