Group 2 trends identify W as strontium Steps:

• Sulfate of Y decomposes slowest (most stable), so thermal stability decreases down group: Y = Mg.
• W reacts faster with HCl than Y = Mg (reactivity increases down group), so W = Ca, Sr, or Ba.
• Hydroxide of X less soluble than W's (solubility increases down group), so X above W; W ≠ Ca (X = Mg taken by Y).
• Remaining: X = Ca (above Sr), W = Sr, Z = Ba satisfies all; W = Ba allows X = Ca or Sr but leads to non-unique fit, ruled out by precise matching. Why C is correct:
• Sr(NO₃)₂ molar mass = 87.6 + 2(62) = 211.6, matches W = Sr from trends. Why the others are wrong:
• A: Mg(NO₃)₂ = 148.3, but Mg = Y (slowest sulfate decomposition).
• B: Ca(NO₃)₂ = 164.1, but Ca cannot be W (no element above for X).
• D: Ba(NO₃)₂ = 261.3, but Ba = Z (remaining after assignment). Final answer: C