The correct option is H+.
Explanation
According to the Arrhenius definition of acids and bases, an acid is a substance that dissociates in an aqueous solution to release hydrogen ions (H+). It is the concentration of these ions that determines the acidic nature of a solution.
Analysis of Options:
- H+ H+ (Hydrogen Ion): Correct. The hydrogen ion is the fundamental species responsible for acidic properties. In water, these ions associate with water molecules to form hydronium ions (H3O+), which impart acidity.
- OH- OH- (Hydroxide Ion): Incorrect. The hydroxide ion is responsible for the basic or alkaline properties of a solution, not acidic properties.
- Na+ Na+ (Sodium Ion): Incorrect. Sodium ions are alkali metal cations. While they are components of salts and bases, they do not independently cause acidity.
- K+ K+ (Potassium Ion): Incorrect. Potassium ions are also alkali metal cations and do not contribute to the acidic character of a solution.
Key Takeaway: The defining characteristic of an acid in an aqueous solution is the production of hydrogen ions (H+), whereas bases are characterized by the production of hydroxide ions (OH-).