Group 2 solubility trends identify X as strontium Steps: - Calcium is in group 2; X is another group 2 element below Ca based on decreasing sulfate solubility and increasing hydroxide solubility. - Sulfate solubility decreases down group 2, so X's sulfate solubility is lower than Ca's 4.6 × 10^{-2} mol/100 g water, matching 7.1 × 10^{-5} mol/100 g water for Sr. - Hydroxide solubility increases down group 2, so X's hydroxide solubility exceeds Ca's 1.5 × 10^{-3} mol/100 g water but remains low, fitting 2.0 × 10^{-3} mol/100 g water for Sr. - Carbonate decomposition temperature (if provided for X > 840°C) confirms X below Ca, aligning with Sr at ~1250°C. Why B is correct: - Matches SrSO₄ solubility (~7.1 × 10^{-5} mol/100 g water) and Sr(OH)₂ solubility (~2.0 × 10^{-3} mol/100 g water) per group 2 trends. Why the others are wrong: - A: Hydroxide solubility (3.0 × 10^{-3}) exceeds Sr value, fitting Ba more closely. - C: Sulfate solubility (1.8 × 10^{-1}) is higher than Ca, indicating element above Ca like Mg. - D: Sulfate solubility (1.8 …