What type of reaction occurs when an acid donates a proton?

When an acid donates a proton, it is engaged in an acid-base reaction. This type of reaction is defined by the transfer of a proton (H⁺ ion) from the acid to a base. According to the Brønsted-Lowry theory, acids are proton donors and bases are proton acceptors. Therefore, when an acid donates a proton, it fulfills the definition of an acid-base reaction, resulting in the formation of a conjugate base and the corresponding increase in the concentration of the conjugate acid in the reaction medium.

In contrast, a neutralization reaction specifically refers to the interaction between an acid and a base that results in the formation of water and a salt; this may occur as part of an acid-base reaction but focuses on the product formation. An oxidation-reduction reaction involves the transfer of electrons between species, which is unrelated to proton transfer. A decomposition reaction, on the other hand, involves a single compound breaking down into two or more simpler substances without any proton transfer being necessary.

Thus, the process of an acid donating a proton is most accurately categorized under acid-base reactions, making this the correct answer.