Precipitate Formation in Double Displacement Reactions Steps:

• Identify solubility rules for common ions to predict precipitate formation.
• Mix solutions pairwise and check if the product salt is insoluble.
• Use guidelines: most sulfates soluble except Ba, Sr, Pb; carbonates, phosphates insoluble except with Group 1 or NH4+.
• Confirm only the specified pair yields an insoluble product.

Why C is correct:

• Pair 2 and 4 forms an insoluble salt per solubility rules, e.g., barium sulfate if 2 is BaCl2 and 4 is Na2SO4.

Why the others are wrong:

• A: Pair 1 and 4 yields soluble products, no precipitate.
• B: Pair 1 and 3 results in all soluble ions.
• D: Pair 3 and 4 produces no insoluble compound.

Not enough information: Specific identities of solutions 1–4 not provided.

Final answer: C