Which combination of ions will yield calcium carbonate precipitate when mixed in aqueous solution?

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Calcium carbonate precipitates when carbonate ions are present to bind with calcium ions, forming CaCO3(s), which is highly insoluble in water. The key idea is that the carbonate ion provides the component that makes the solid, and the solid only forms once the product of the calcium and carbonate ion concentrations exceeds CaCO3’s very small solubility product. Among the options, only the pairing that includes carbonate supplies the needed ion; pairing calcium with chloride or nitrate creates soluble salts that stay dissolved, and pairing with sulfate would give calcium sulfate, not calcium carbonate. So the combination that yields the calcium carbonate precipitate is calcium ions with carbonate ions.