What defines a redox reaction?

A redox reaction is characterized by the simultaneous occurrence of oxidation and reduction processes. In such a reaction, one species loses electrons and is oxidized, leading to an increase in its oxidation state, while another species gains those electrons and is reduced, resulting in a decrease in its oxidation state. This exchange of electrons is the fundamental concept that defines redox reactions, highlighting the interdependent nature of oxidation and reduction processes.

The other options do not accurately define redox reactions. For instance, a reaction that involves only oxidation or only reduction would not meet the criteria for a redox process, as both these actions need to occur together for the reaction to be classified as redox. Additionally, a reaction with no change in oxidation states would imply that no electron transfer or changes in oxidation state are taking place, which is contrary to the essence of a redox reaction. Thus, the definition emphasizing the simultaneous occurrence of both oxidation and reduction accurately captures what a redox reaction entails.