What does a reducing agent do?

A reducing agent is characterized by its ability to donate electrons in a redox reaction, leading to the reduction of another substance involved in the process. In the context of redox (reduction-oxidation) reactions, reduction refers to the gain of electrons or decrease in oxidation state by a chemical species. When a reducing agent donates electrons, it itself is oxidized, meaning it undergoes an increase in its oxidation state.

For instance, in the reduction of metal ions to their elemental form, the reducing agent donates electrons to these ions, thereby converting them to a lower oxidation state (the elemental state). This role is fundamental in many chemical reactions, including those in biological systems and industrial processes where electron transfer is pivotal to creating compounds or energy.

The other options each describe different chemical behaviors that do not pertain specifically to reducing agents. Raising activation energy and accelerating reaction rates relate more to catalysts and their roles in influencing the speed of chemical reactions, but they do not involve the transfer of electrons or changes in oxidation states. Therefore, the focus on electron donation and the reduction of other substances distinctly marks option A as the correct choice concerning the definition and function of a reducing agent in redox chemistry.