A metal that forms soluble trioxosulphate(IV) ion is

Correct answer: Potassium

Explanation: The trioxosulphate(IV) ion is the sulphite ion, SO₃²⁻. By solubility rules, Group I (alkali) metal salts (e.g., sodium, potassium) are generally soluble, including their sulphites. In contrast, many sulphites of alkaline-earth and transition metals are sparingly soluble or insoluble.

• Barium — Forms BaSO₃, largely insoluble (wrong).
• Potassium — Correct. K₂SO₃ is soluble.
• Manganese — MnSO₃ is insoluble (wrong).
• Aluminium — Aluminium “sulphite” hydrolyses; not a typical soluble sulphite (wrong).