Final answer:
The most soluble substance in water among the given options is 2-butanol (C₄H₉OH), an alcohol.
Step-by-step explanation:
Water is considered a polar solvent. Substances that dissolve in water are typically polar or ionic compounds. Among the given options, the most soluble substance in water would be 2-butanol (C₄H₉OH). 2-butanol is an alcohol, and alcohols are covalent compounds that dissolve in water to give solutions of neutral molecules. Copper (II) sulfate (CuSO₄) is also soluble in water, but it is an ionic compound, not a covalent compound like 2-butanol.