Final answer:
KOH (potassium hydroxide) is the substance that will form a base when dissolved in water because it releases hydroxide ions (OH-) into the solution.
Step-by-step explanation:
Among the substances listed, KOH (potassium hydroxide) will form a base when dissolved in water. A base is defined as a substance that either releases hydroxide ions (OH-) in solution or accepts H+ from the solution. KOH, being an ionic compound composed of an alkali metal cation and hydroxide ion, dissolves in water and dissociates to produce K+ and OH- ions, thus increasing the number of hydroxide ions in the solution and making it basic.
Other substances like HBr (hydrobromic acid), SO₂ (sulfur dioxide), and HCl (hydrochloric acid) are acids and will release H+ ions when dissolved in water, thus increasing the solution's acidity rather than its basicity.