Which of the following is a common laboratory strong acid?

Hydrochloric acid (HCl) is a common strong acid used in laboratories. It fully dissociates in water, meaning it breaks down completely into hydrogen ions (H⁺) and chloride ions (Cl⁻), resulting in a high concentration of H⁺ ions in solution. This complete dissociation is what characterizes a strong acid, as it greatly increases the acidity of the solution and allows for the acid to effectively react with other substances.

In contrast, the other options provided do not qualify as strong acids in a laboratory context. Water (H₂O) is neutral and does not exhibit acidic properties. Acetic acid (CH₃COOH) is a weak acid, which means it only partially dissociates in solution, resulting in fewer hydrogen ions. Ammonium ion (NH₄⁺), while it can react as an acid in specific contexts, is also not classified as a strong acid. Therefore, HCl stands out as the common strong acid among the choices listed, making it the correct answer.