Halogen trends with acids and displacement reactions

Steps:

• Recall that halogen reactivity decreases down Group 17, so astatine is the least reactive.
• For silver halides, solubility in ammonia decreases down the group; AgAt is insoluble like AgI.
• Displacement reactions favor more reactive halogens displacing less reactive ones below them.
• Concentrated H2SO4 oxidizes heavy halides (I-, At-) to free halogens via redox.

Why D is correct:

• Concentrated H2SO4 acts as an oxidizing agent, oxidizing At- to At2: 2NaAt + 3H2SO4 → 2NaHSO4 + At2 + 2H2O + SO2.

Why the others are wrong:

• A: AgAt, like AgI, is insoluble in dilute ammonia due to strong lattice energy.
• B: Astatine cannot displace Cl- as it is less reactive than chlorine.
• C: Dilute H2SO4 forms HAt(aq) with NaAt, but no oxidation to free astatine occurs.

Final answer: D