Final answer:
Oxidation is the correct answer as it involves the diffusion of hair pigment by converting primary intermediates into larger dye molecules that lock inside the hair, resulting in a permanent color change.
Step-by-step explanation:
Oxidation is a chemical process involving the diffusion of the natural hair color pigment or artificial hair color from the hair. During permanent hair coloring, three main components are used: a primary intermediate, a coupler, and an oxidant. The process unfolds in a series of steps where the primary intermediate, often a derivative of diaminobenzene or diaminotoluene, is oxidized to a quinone state. This intermediate then reacts with a coupler compound and is further oxidized to form the final dye. This final dye is a larger molecule that can be locked inside the hair, imparting a permanent color change.
Contrary to oxidation, the reduction reaction is often involved in hair treatments such as perming, where reducing agents are used to disrupt disulfide bonds in the hair's keratin structure. In the context of hair coloring, oxidation is the correct answer among the choices given because it is during this step that the dye molecules form and bond within the hair.