Which statement best defines an "Acid"?

An acid is best defined by its ability to donate protons (H⁺ ions) in a solution, which is exemplified by substances like hydrochloric acid (HCl). Hydrochloric acid is a strong acid commonly found in gastric juice and is known for its highly acidic properties. It dissociates completely in water to yield H⁺ and Cl⁻ ions, demonstrating the characteristic property of acids to increase the concentration of protons in a solution, thereby lowering the pH.

The other options describe characteristics of acids or bases but do not accurately define what an acid truly is. For example, a base is defined differently as it produces hydroxide ions (OH⁻), having the opposite effect of acids. The production of hydroxide ions is not a property of acids. Additionally, a high pH indicates a basic solution rather than an acidic one, and thus does not accurately characterize acids.