Solubility rules for ionic compounds Steps:

• Apply solubility rules: sodium salts and nitrates are always soluble in water.
• Carbonates are insoluble except with group 1 cations like sodium.
• Identify white insoluble compounds: calcium carbonate (CaCO₃) fits, as it's white and insoluble.
• Match options to rules: select combination avoiding soluble ions while allowing insoluble pairs.

Why C is correct:

• Lists calcium (✓) and carbonate (✓) as present, forming CaCO₃, which is insoluble per general solubility rules for carbonates with group 2 metals.

Why the others are wrong:

• A: Sodium (✓) and nitrate (✓) form NaNO₃, soluble in water.
• B: Marks sodium (✗) and nitrate (✗) but includes calcium (✓) and carbonate (✓), yet implies absence prevents insolubility—contradicts rules.
• D: Includes soluble ions like nitrate (✓), leading to a soluble compound.

Final answer: C