Identifying elements Q and R from properties

Steps:

• Sulfate of Q insoluble identifies Q as barium (Ba, proton number 56); most group 2 sulfates are insoluble except Mg and Ca.
• Element R is a gas at RTP with likely halogen outer shell (7 electrons), matching chlorine (relative atomic mass 35.5).
• Choices pair proton number (likely for Q) with relative atomic mass (likely for R).
• Diagram (not provided) would confirm outer shells: Q with 2 valence electrons (group 2), R with 7 (group 17).

Not enough information without diagram to confirm shells, but assuming standard match for D.

Why D is correct:

• Pairs Ba proton number 56 (insoluble sulfate) with Cl atomic mass 35.5 (gas), fitting properties.

Why the others are wrong:

• A: 12 is Mg (soluble sulfate, not Q).
• B: 80 not a gas atomic mass (Kr ≈84, but doesn't fit shell).
• C: 96 is curium (radioactive solid, irrelevant).

Final answer: D