An insoluble salt can be prepared by

Correct answer: Double decomposition

Explanation: Insoluble salts are typically prepared by precipitation—mixing two soluble salts so that an insoluble product forms and separates as a solid. Example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). This method is called double decomposition (or metathesis).

• The reaction of trioxocarbonate(V) with an acid — Usually gives a soluble salt, CO₂, and water (not a reliable way to get an insoluble salt).
• Double decomposition — Correct. Two soluble salts react to precipitate an insoluble salt.
• The action of dilute acid on an insoluble base — Produces a soluble salt solution and water (e.g., CuO + H₂SO₄ → CuSO₄(aq)).
• The reaction of metals with an acid — Yields a soluble salt and hydrogen gas; not a precipitation method.