Here's why:

* Solubility and Temperature: The solubility of most solids in liquids increases as the temperature increases. This is because higher temperatures provide more kinetic energy to the molecules, allowing them to break apart and dissolve more easily.

* KCl's Solubility: KCl is an ionic compound, and its solubility is significantly affected by temperature. While KCl is soluble in water at room temperature, its solubility increases drastically as the water gets hotter.

Therefore, the temperature at which KCl would be most soluble is the highest temperature at which the water remains liquid (its boiling point).