Magnesium (Mg) is a reducing agent because it is oxidized during the reaction. Since a substance that is oxidized (loses electrons) is the reducing agent, while the substance that is reduced (gains electrons) is the oxidizing agent.

In this specific reaction, the oxidation state of magnesium changes from 0 (in Mg(s)) to +2 (in MgCl₂(aq)), an increase in oxidation state, which means it loses electrons and is oxidized.

B. reduced: A substance that is reduced is the oxidizing agent, not the reducing agent. Hydrogen ions (H+) are reduced in this reaction (from +1 to 0).

C. a metal: While magnesium is a metal, being a metal is its physical and chemical classification, not the reason it acts as a reducing agent in the context of a specific redox reaction. The reason is rooted in the change in its oxidation state.

D. a solid: The state of magnesium (solid) is a physical property. The chemical reason it acts as a reducing agent is its tendency to lose electrons (get oxidized) during the reaction.

:. The correct answer is oxidized - option A