When an acid dissolves in water, hydrogen ions are released. Therefore, the correct answer is option A.
Here’s why: Acids are substances that increase the concentration of hydrogen ions (H+) in a solution when they are dissolved in water. This process occurs because the acid donates protons (H+) to the water molecules, resulting in the formation of hydronium ions (H3O+). This increase in hydrogen ion concentration is what characterizes an acidic solution. The other options either involve the release of different substances or do not happen in a typical acid-base reaction. Therefore, the release of hydrogen ions is a key characteristic of acids in aqueous solutions.