Final answer:
The correct answer is A) Gaining electrons, reducing, oxidizing. Reduction refers to the gain of electrons by a reactant, which means its oxidation number decreases in the course of a redox reaction.
Step-by-step explanation:
If a reactant is being reduced, it is undergoing a chemical change where it gains electrons. During this process, the oxidation number of the reactant decreases. Referring to the provided options, the correct answer is A) Gaining electrons, reducing, oxidizing, where 'reducing' describes the process happening to the reactant itself and 'oxidizing' refers to the overall reaction that involves another species being oxidized. It's important to note that reduction always occurs alongside oxidation in what are known as redox (reduction-oxidation) reactions.