Precipitation of Insoluble Calcium Carbonate Steps:

• River water contains NH₄⁺, HCO₃⁻, Cl⁻, and NO₃⁻ ions; calcium hydroxide (Ca(OH)₂) dissociates to Ca²⁺ and OH⁻.
• OH⁻ reacts with HCO₃⁻ to form CO₃²⁻ and H₂O: HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O.
• Ca²⁺ combines with CO₃²⁻ to form CaCO₃, which is insoluble and precipitates.
• Other ions (NH₄⁺, Cl⁻, NO₃⁻) form soluble compounds with Ca²⁺ or OH⁻, so no other precipitates occur.

Why B is correct:

• CaCO₃ has low solubility (Ksp = 3.3 × 10⁻⁹), forming a precipitate per solubility product rules.

Why the others are wrong:

• A: CaCl₂ is highly soluble in water.
• C: Ca(NO₃)₂ is soluble, used in fertilizers.
• D: NH₄OH is a soluble weak base, not a solid precipitate.

Final answer: B