D And F Block Elements

Given below are two statements: one is labelled as "Assertion A" and the other is labelled as "Reason R"

Assertion A : In the complex \(\mathrm{Ni}(\mathrm{CO})_{4}\) and \(\mathrm{Fe}(\mathrm{CO})_{5}\), the metals have zero oxidation state.

Reason R : Low oxidation states are found when a complex has ligands capable of \(\pi\)-donor character in addition to the $\sigma$-bonding.

In the light of the above statements, choose the most appropriate answer from the options given below