Which of the following is an example of a strong acid?

Hydrochloric acid (HCl) is classified as a strong acid because it completely dissociates in water, releasing all of its hydrogen ions (H⁺) into the solution. This complete ionization is what characterizes a strong acid, resulting in a high concentration of H⁺ ions, which leads to a lower pH and a greater acidity in the solution.

In contrast, sodium hydroxide is a strong base, not an acid. Acetic acid and carbonic acid are both weak acids, as they do not fully dissociate in solution. Acetic acid, for instance, only partially ionizes, which means it does not contribute a maximal amount of H⁺ ions to the solution. Similarly, carbonic acid only partially dissociates into bicarbonate and hydrogen ions, further demonstrating its weakness as an acid.