What type of reaction involves a proton transfer?

The correct answer is a reaction that involves a proton transfer, which characterizes acid-base reactions. In these reactions, an acid donates a proton (H⁺) to a base, which accepts it. This fundamental concept is central to many chemical interactions and is defined by the Brønsted-Lowry theory of acids and bases. For instance, in the reaction between hydrochloric acid (HCl) and ammonia (NH₃), HCl donates a proton to NH₃, forming ammonium ion (NH₄⁺) and chloride ion (Cl⁻), thereby illustrating the transfer of a proton.

In contrast, addition reactions involve the combined reactivity of certain molecules where new bonds are formed, typically in organic chemistry, without the explicit transfer of protons. Synthesis reactions also focus on the combination of reactants to form a product but do not specifically involve proton transfers. Redox reactions center on the transfer of electrons between species, which does not directly correlate with proton transfer; instead, they focus on changes in oxidation states. Thus, the defining feature of a proton transfer firmly identifies acid-base reactions as the correct choice.