What is the role of a reducing agent?

A reducing agent is a substance that donates electrons to another substance, causing that substance to be reduced while the reducing agent itself is oxidized. In the context of the options provided, the correct answer identifies a specific chemical process involving the reduction of a metal oxide.

When heating a metal oxide with carbon monoxide, the carbon monoxide acts as the reducing agent. It donates electrons to the metal oxide, thereby reducing the metal ions in the oxide, generating a pure metal, and converting carbon monoxide into carbon dioxide. This is a common method in metallurgy for extracting metals from their ores, illustrating the practical application of a reducing agent in chemical reactions.

The other options do not accurately describe the role of a reducing agent. For instance, oxidizing a metal oxide does not involve a reducing agent, and liberating hydrogen gas or creating salts do not directly correlate with the function of a reducing agent in the context of reactions involving metal oxides.