Redox reactions involve electron transfer between species

Steps:

• Define redox as reactions where oxidation (electron loss) and reduction (electron gain) occur simultaneously.
• Analyze A: Br₂ + H₂ → 2HBr; H oxidized (0 to +1), Br reduced (0 to -1), confirming redox.
• Analyze B: Br⁻ + conc. H₂SO₄ → Br₂ + SO₂ + H₂O; Br⁻ oxidized (-1 to 0), S reduced (+6 to +4), confirming redox.
• Analyze C: KI + AgNO₃ → AgI + KNO₃; precipitation with no oxidation state changes, not redox.

Why D is correct:

• D selects A and B, matching the definition of redox via observed electron transfers in those mixtures.

Why the others are wrong:

• A selects only bromine + hydrogen, missing B's redox.
• B selects only bromide + sulfuric acid, missing A's redox.
• C selects potassium iodide + silver nitrate, which is double displacement without electron transfer.

Final answer: D