What defines a reducing agent?

A reducing agent is defined by its ability to donate electrons or to remove oxygen from another substance during a chemical reaction. This action effectively reduces the oxidation state of the substance that gains the electrons, while the reducing agent itself is oxidized in the process.

In this context, when we say a reducing agent "takes oxygen or gives electrons," it highlights that it facilitates the reduction process by either donating electrons to an electron-accepting species or stripping away oxygen from compounds. Therefore, this characterization captures the essence of what a reducing agent does in redox reactions.

The other options do not accurately relate to the definition of a reducing agent. A substance that oxidizes other materials would be more accurately classified as an oxidizing agent, as it facilitates the gain of oxygen or loss of electrons from another substance. Similarly, rusting involves both oxidation and reduction processes but cannot singularly define a reducing agent since it describes a broader phenomenon. Increasing energy levels does not relate to redox chemistry and is not a function of reducing agents.