Redox reactions involve electron transfer or oxidation state changes

Steps:

• Recall that redox reactions require simultaneous oxidation and reduction, detectable by changes in oxidation numbers.
• Examine each pair: identify the reaction type and check for electron transfer.
• For option A, CH₃CH₂Br reacts with a reducing agent (e.g., implied metal like Zn), involving Br reduction and carbon oxidation state shift.
• Confirm only A shows clear redox characteristics among options.

Why A is correct:

• CH₃CH₂Br undergoes dehalogenation or coupling with metals, where Br gains electrons (reduction) and alkyl group loses (oxidation), per redox definition.

Why the others are wrong:

• B: CH₃CH₂OH + H₃PO₄ is acid-catalyzed dehydration to ethene, an elimination without oxidation state change.
• C: CH₃COOH + HCN forms no standard reaction; it's not redox, lacking electron transfer.
• D: HCO₂CH₃ + H₂SO₄ is acid hydrolysis to methanol and formic acid, a substitution without redox.

Final answer: A