Which one of the following bases completely ionizes in an aqueous solution?

The correct choice reflects NaOH, which is a strong base that completely dissociates in aqueous solutions. When NaOH is dissolved in water, it breaks down into sodium ions (Na⁺) and hydroxide ions (OH⁻) without leaving any undissociated molecules behind. This complete ionization is characteristic of strong bases, as they have a tendency to readily release hydroxide ions into the solution, contributing to an increase in the pH of the solution.

In contrast, the other options presented do not fully ionize in water. Acetic acid (CH3COOH) is a weak acid and only partially dissociates, leading to an equilibrium between its ions and molecules. Ammonium hydroxide (NH4OH) is also a weak base, as it does not completely dissociate into ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). Additionally, water (H2O) can act both as an acid and a base but does not ionize completely in the context of this question; rather, it exists largely as whole water molecules and can only produce a small amount of ions through self-ionization.

Thus, NaOH's complete ionization makes it the clear choice among