Which of the following is considered a weak base?

A weak base is a substance that does not completely ionize in solution, resulting in a relatively low concentration of hydroxide ions compared to strong bases. In the options provided, ammonia (NH₃) is classified as a weak base because when it dissolves in water, it partially reacts with water molecules to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). However, the extent of this ionization is limited, which is characteristic of weak bases.

On the other hand, the other substances listed are classified as strong bases or acids. Potassium hydroxide (KOH) and sodium hydroxide (NaOH) are strong bases that fully dissociate in water, releasing a significant amount of hydroxide ions. Hydrochloric acid (HCl) is a strong acid that fully ionizes in solution, generating a high concentration of hydrogen ions (H⁺). Thus, among the choices, ammonia is the only one that qualifies as a weak base due to its incomplete ionization in water.