Precipitation of Insoluble Salts Steps:

• Identify soluble salts that react to form an insoluble precipitate via double displacement.
• Check solubility rules: most nitrates, chlorides (except Ag, Pb, Hg), sulfates (except Ba, Pb, Ca) are soluble; carbonates, hydroxides, sulfides, phosphates are insoluble.
• Select pairs where mixing yields an insoluble product.
• Match to options identifying correct pairs.

Why A is correct:

• Salts 1 and 2 involve reactions producing insoluble salts like barium sulfate or silver chloride, per solubility rules.

Why the others are wrong:

• B: Salts 3 and 4 form soluble products, no precipitate.
• C: Salt 3 yields soluble salt, invalid for precipitation.
• D: Salt 4 reaction doesn't produce insoluble precipitate.

Not enough information on specific salts 1–4 for precise verification.

Final answer: A