Answer is (3) - Pb²⁺.
Explanation;
Among the given choices PbCl₂ is insoluble in water. But it can be soluble in hot water. It appears as white precipitate. The Ksp of PbCl₂ is very small. That's why it is insoluble in water.
Fe²⁺ + 2Cl⁻ → FeCl₂ (soluble in water. Forms pale green color solution)
Mg²⁺ + 2Cl⁻ → MgCl₂ (soluble in water. Forms clear colorless solution)
Pb²⁺ + 2Cl⁻ → PbCl₂ (poorly soluble in water. Forms white precipitation)
Zn²⁺ + 2Cl⁻ → ZnCl₂ (soluble in water. Forms clear colorless solution)
