What to add from an aqueous solution of Br- to displace Br2?

You need something that is more oxidizing than Br2 to displace it from solution of Br(-). Since oxidizing power of halogens increases as you move up the group in the Periodic Table, either Cl2 or F2 will displace Br2 from Br(-). The correct answer is (b) Cl2(aq)