Which of the following is a strong base?

KOH, or potassium hydroxide, is indeed recognized as a strong base. Strong bases are substances that completely dissociate in water, producing hydroxide ions (OH⁻) which make the solution strongly alkaline. When KOH is dissolved in water, it breaks down into its constituent ions: potassium ions (K⁺) and hydroxide ions (OH⁻), resulting in a high concentration of hydroxide ions that significantly increase the pH of the solution.

In contrast, HF, or hydrofluoric acid, is a weak acid, as it does not fully dissociate in water. H2O, or water, is neutral and does not function as a strong base or acid under typical conditions. C6H5OH, or phenol, while it has acidic properties, is also not classified as a strong base and exhibits more of a weakly acidic behavior.

Thus, KOH stands out as a strong base due to its complete dissociation in aqueous solution, providing a clear increase in the concentration of hydroxide ions.