Which of the following reactions is categorized as spontaneous?

Spontaneous reactions are those that occur without the need for external energy input and are characterized by a negative change in Gibbs free energy (ΔG < 0). In this context, exergonic reactions, which are associated with a release of energy, fit this definition perfectly. These reactions proceed on their own because the products have less free energy than the reactants, resulting in a negative ΔG.

The term "endergonic" refers to reactions that require energy input to proceed, resulting in a positive ΔG, meaning they are non-spontaneous unless energy is provided. Anabolic reactions, which build complex molecules from simpler ones, are typically endergonic and therefore also not spontaneous. The option suggesting that all reactions are spontaneous is inaccurate, as spontaneous and non-spontaneous reactions exist on a spectrum depending on the thermodynamic characteristics, especially ΔG.

Thus, the correct categorization of spontaneous reactions aligns with exergonic processes, as they naturally move toward equilibrium without external energy requirement, showcasing why they are the definitive choice in this scenario.