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 $$\pi$$-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

In $$\text{Ni(CO)}_4$$ and $$\text{Fe(CO)}_5$$, the CO ligands are neutral, so the overall charge on each complex equals the charge on the metal. Since both complexes are uncharged, the metals are in the zero oxidation state. Assertion A is correct.

Reason R states that low oxidation states arise when ligands have $$\pi$$-bonding character beyond $$\sigma$$-bonding. CO forms a synergic bond: it donates a lone pair to the metal ($$\sigma$$-bond) and simultaneously accepts electron density from the metal's filled $$d$$-orbitals into its empty $$\pi^*$$ orbitals ($$\pi$$-back-bonding). This $$\pi$$-interaction stabilises the metal in a low oxidation state. Reason R correctly identifies this $$\pi$$-bonding ability as the cause of the low oxidation state, and it explains Assertion A.

Both A and R are correct, and R is the correct explanation of A. The answer is $$\boxed{\text{Option (D)}}$$.