Which of the following changes requires an oxidizing agent? 2h+ yields h2 so3 yields so42- 2br- yields br2 mno2 yields mn2+

An oxidizing agent is one that oxidizing the other chemical specie while the oxidizing agent itself if reduced. During oxidation, the oxidation number of the chemical specie being oxidized increases. 
In the reaction:
2Br- → Br₂
This is because the oxidation number of bromine is -1 before the reaction and 0 after the reaction.