What defines a reagent in chemistry?

A reagent in chemistry is defined as a substance that facilitates a chemical reaction. This characterization is essential because reagents are usually involved in either driving a reaction forward or being transformed during the reaction. They can provide essential atoms or functional groups to reactants and participate directly in the process, whether through a chemical transformation, catalysis, or other means.

In contrast, the other options present substances that do not fulfill the role of a reagent. An inert compound that stabilizes reactions may provide some level of support, but it does not actively participate in a chemical change. A product resulting from a reaction is the outcome of the reagent's action and represents what is formed after the reaction has occurred, rather than a participant in it. A solvent used in extractions serves to dissolve substances, aiding in the separation process, but is not directly involved in the chemical transformation itself. Therefore, the definition that encompasses the function and importance of a reagent is accurately captured by the description of a substance that facilitates a chemical reaction.