Which of the following compounds can act as a weak base?

Ammonia is classified as a weak base due to its ability to partially ionize in water, forming ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). In an aqueous solution, ammonia does not completely dissociate like strong bases such as sodium hydroxide, which fully ionizes to provide hydroxide ions. Therefore, the equilibrium between ammonia, ammonium ions, and hydroxide ions illustrates its weak basicity:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.

This equilibrium demonstrates that ammonia can accept protons from water, thereby behaving as a base, but the extent of its ionization is limited, characteristic of weak bases.

In contrast, sodium hydroxide and potassium chloride are not relevant as weak bases in this context; sodium hydroxide is a strong base that completely dissociates in solution, while potassium chloride is a neutral salt that does not exhibit basic properties. Sulfuric acid is a strong acid that, when dissolved in water, dissociates fully to release protons and thus behaves as an acid rather than a base. Hence, ammonia stands out as the compound that effectively acts as a weak base.