Which of the following salts shows the lowest solubility in water? (Ksp values: Ag₂S = 1.6 × 10⁻⁴⁹; Bi₂S₃ = 1.0 × 10⁻⁷²; HgS = 1.6 × 10–54; Mg(OH)₂ = 8.9 × 10⁻¹²; MnS = 2.3 × 10⁻¹³)