Nitrate thermal stability identifies Q as calcium; R is the hydroxide from successive reactions.

Steps:

• Group 2 nitrates increase in thermal stability down the group: Ca(NO₃)₂ decomposes below Sr(NO₃)₂, while Ba(NO₃)₂ is more stable.
• Q forms a nitrate less stable than Sr(NO₃)₂, so Q is calcium.
• Flow diagram: first reaction is Q + O₂ → QO (oxide formation).
• Second reaction: QO + H₂O → Q(OH)₂, so R is Ca(OH)₂.

Why B is correct:

• Ca(OH)₂ forms via CaO + H₂O → Ca(OH)₂, the standard reaction for Group 2 oxides with water.

Why the others are wrong:

• A. CaO is the intermediate from the first reaction, not R.
• C. BaO is barium oxide, but Q is calcium.
• D. Ba(OH)₂ is barium hydroxide, but Q is calcium.

Final answer: B