Which of the following describes an acid?

An acid is defined by its ability to release hydrogen ions (H⁺) when dissolved in water. This characteristic is fundamental to the behavior of acids in chemical reactions and their interaction with bases. When an acid dissolves in water, it ionizes to produce H⁺ ions, which increases the concentration of these ions in the solution and lowers the pH, indicating a more acidic solution.

This property is what distinguishes acids from other substances. For instance, the release of hydroxide ions (OH⁻) is a defining feature of bases, not acids. Additionally, a solution with a pH greater than 7 is considered basic (alkaline), while a neutral solution has a pH of 7, further demonstrating that these options do not accurately describe acids. Therefore, the correct identification of an acid centers on its ability to donate H⁺ ions in water, making option C the best choice.