All four options are anions that usually form soluble compounds:
- Hydroxide ions (OH⁻)
- Nitrate ions (NO₃⁻)
- Sulfate ions (SO₄²-)
- Carbonate ions (CO₃²-)
What are these anions?
Hydroxide ions (OH⁻): Most hydroxides are soluble, with a few exceptions like magnesium hydroxide and calcium hydroxide.
Nitrate ions (NO₃⁻): Almost all nitrates are readily soluble in water.
Sulfate ions (SO₄²⁻): The majority of sulfate salts are soluble in water. Exceptions include barium sulfate, calcium sulfate, and lead sulfate.
Carbonate ions (CO₃²⁻): Most carbonate salts are soluble, except for those of alkaline earth metals like calcium and magnesium.
Therefore, all four options contribute to forming soluble compounds in most cases.