Answer
Better oxidizing agents are
$ Au^{3+}(aq) $
$ Ag^{+}(aq) $
$ Cr^{3+}(aq) $
$ O_{2(g)}$(In acid)
Work Step by Step
In order to find a better oxidizing agent, we must know the standard reduction potential values of the species involved. The one with a higher reduction potential value acts as a better oxidizing agent.
$ Br_{2(g)} → Br^{-}(aq)$ $ E^{\circ} = +1.07 V $
$ Au^{3+}(aq) → Au_{(s)} $ $ E^{\circ} = +1.50 V $
Comparing $ Br_{2(g)}$ and $ Au^{3+}(aq) $, $ Au^{3+}(aq) $ is a better oxidizing agent.
Comparing $ H_{2(g)}$ [0.00 V] and $ Ag^{+}(aq) $[ +0.80 V], $ Ag^{+}(aq) $ is a better oxidizing agent.
Comparing $ Cd^{2+}(aq)$ [-0.40 V] and $ Cr^{3+}(aq) $[ +1.33 V], $ Cr^{3+}(aq) $ is a better oxidizing agent.
Comparing $ O_{2(g)}$(In acid) [1.23 V] and $ O_{2(g)}$(In basic) [0.40 V], $ O_{2(g)}$(In acid) is a better oxidizing agent.
