Which of the following is a characteristic of an acid?

Acids are defined by their ability to donate protons (H⁺ ions) to other substances. This property is fundamental to the Brønsted-Lowry definition of acids and bases, which characterizes acids specifically as proton donors. When an acid donates a proton, it can lead to the formation of a corresponding conjugate base. This interaction is key to numerous chemical reactions, particularly in aqueous solutions where the behavior of acids is most commonly observed.

The other options present characteristics associated with bases or chemical reactions resulting from the interaction of acids and bases. Bases, for example, are characterized by their production of hydroxide ions (OH⁻), which is why acids do not produce these ions directly. A pH greater than 7 is indicative of basic solutions, while a pH less than 7 is associated with acidic solutions. Lastly, while acids can react with bases to produce salts, this is a result of their proton-donating ability rather than a defining characteristic of acids themselves. Therefore, describing acids predominantly as proton donors effectively captures an essential aspect of their chemical nature.