What ion must an acid donate in order to function as an acid?

An acid functions by donating protons, which are represented by hydrogen ions (H+). When an acid donates an H+ ion, it increases the concentration of free protons in a solution, which leads to an increase in acidity. This characteristic is fundamental to the Arrhenius and Brønsted-Lowry definitions of acids.

In contrast, the other listed ions do not fulfill the role of an acid. Hydroxide ions (OH-) relate to basic properties rather than acidic ones. Sodium ions (Na+) and chloride ions (Cl-) are generally neutral and do not participate in the proton donation that defines acidic behavior. Therefore, the key to understanding why the correct answer is H+ lies in recognizing that the defining feature of acids is their ability to donate protons, thus creating either hydronium ions (H3O+) in aqueous solution or increasing the overall concentration of H+.