Which of the following represents a weak acid?

The choice that represents a weak acid is acetic acid, denoted as CH3COOH. Acetic acid only partially dissociates in solution, which means that at any given time, a significant amount of the acid remains in its molecular form rather than fully converting to ions. This partial ionization is characteristic of weak acids, which means they do not yield a high concentration of hydrogen ions (H⁺) in solution compared to strong acids.

In contrast, the other options provided are strong acids. Hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3) fully dissociate in solution, producing a high concentration of ions. Because of their ability to completely dissociate, they are classified as strong acids. Therefore, acetic acid is the only one listed that demonstrates the behavior typical of a weak acid, as it establishes an equilibrium between the undissociated molecules and the ions in solution.