What characterizes an acid?

A characteristic of acids is that they donate protons (H⁺ ions) in aqueous solutions, which aligns with option D. When an acid dissolves in water, it increases the concentration of hydrogen ions in the solution, leading to a pH that is typically less than 7. This behavior is fundamental to the definition of an acid in Brønsted-Lowry theory, which emphasizes the role of proton donation in acid-base reactions.

Additionally, the lower pH signifies a higher concentration of hydronium ions (H₃O⁺), reinforcing the notion that acids are proton donors. The indication of a pH less than 7 confirms that the substance is acidic in nature. Since proton donation is a defining feature of acids, this makes the option a correct identification of their characteristic behavior.