In the reaction 3MnO2(s) + 4Al(s)→3Mn(s) + 2Al2O3(s) the oxidising agent is
MnO2
An oxidizing agent is referred to as a chemical compound that readily transfers oxygen atoms to other substance or a substance that gains electrons in a redox reaction. In the reaction, 3MnO2(s) + 4Al(s) → 3Mn(s) + 2Al2O3(s) , MnO2 behaves as an oxidising agent as it oxidises Al to Al2O3