Answer:
Ba⁺², Sr⁺², Ca⁺², and Ra⁺²
Explanation:
If a precipitate is formed, then the ionic compound (a salt or a base) will not be soluble in water. That compound is formed by a metal cation and the anions of the substances given: Cl⁻, SO₄⁻² and OH⁻. With Cl⁻ and SO₄⁻² it will form a salt, and with OH⁻ it will form a base.
The insoluble substance is formed only with SO₄⁻², and most compounds formed with this anion are soluble, the exceptions are the cations Ba⁺², Sr⁺², Ca⁺², Ra⁺², and Pb⁺². The salt formed between Pb⁺² and Cl⁻ is also insoluble, so, Pb⁺² can't be one of the cations.
The soluble base is the ones formed by metals from the groups I and II, all the others are insoluble and would form a precipitated. Ba, Sr, Ca and Ra are from group 2, so they form soluble bases.
Then, the possible cations are: Ba⁺², Sr⁺², Ca⁺², and Ra⁺².
