What does the term "reducing agent" refer to?

The term "reducing agent" refers to a substance that donates electrons during a chemical reaction, leading to the reduction of another substance. In the context of the answer given, when a substance acts as a reducing agent, it is itself oxidized in the process. This means that it loses electrons, which allows another substance to gain those electrons and undergo reduction.

For instance, in a redox reaction, the reducing agent will provide the electrons needed for the reduction of another species while transitioning to a more positive oxidation state itself. This interplay of oxidation and reduction is fundamental to understanding redox reactions, where the substance that gets reduced is effectively accepting the electrons lost by the reducing agent.

The other choices do not accurately capture the essence of a reducing agent. A substance that gains electrons refers to an oxidizing agent, while a substance that donates protons would describe an acid rather than a reducing agent. Lastly, a substance that is reduced is not performing the role of a reducing agent, but rather the opposite; it is the one receiving electrons in the reaction. Thus, the correct understanding of a reducing agent is that it is indeed a substance that is oxidized itself while facilitating the reduction of another.