Which of the following best describes a redox couple?

A redox couple refers to a pair of two species that undergo oxidation and reduction reactions in a redox process. In the context of electrochemistry, a redox couple is specifically composed of a species that acts as the oxidizing agent and its corresponding species that acts as the reducing agent.

The oxidizing agent is the species that gains electrons during the redox reaction (is reduced), while the reducing agent is the species that loses electrons (is oxidized). Therefore, the redox couple illustrates the relationship between these two half-reactions, making A the best description of a redox couple.

Other options do not accurately represent what a redox couple entails. Combining reactions generally refer to processes where reactants combine to form products rather than focusing on the electron transfer aspect of redox reactions. Describing only reduction reactions excludes the vital oxidation component that is integral to the concept of redox couples. Thus, the correct portrayal of a redox couple is the pairing of both the oxidizing and reducing agents.