Answer:
The oxygen gas
Explanation:
Oxidation occurs when one of the reactants loses electrons to become more positively charged. In the initial reaction 4Al is just aluminum metal, which has no charge. In the synthesis reaction, the oxygen causes the aluminum to lose electrons, oxidizing it, as it now has an Oxidation number of +3. Since the oxygen is what causes the Oxidation of aluminum, it is the oxidizing agent.
