HBr and HI can reduce H₂SO₄, HCl can reduce KMnO₄, and HF can reduce: 

HI and HBr are strong reducing agents because, they get easily oxidized and reduce H₂SO₄. On the other hand, HCl is a weak reducing agent, because of the electronegativity of Cl, but gets oxidized in the presence of a strong oxidizing agent. KMnO₄ is a strong oxidizing agent, hence it oxidizes Cl. F has the smallest size among halogens and has a strong electronegativity, so it always tends to get reduced instead of getting oxidized. Therefore, HF is a poor reducing agent and a good oxidizing agent.