An oxidising agent either gains electron or oxidises other substances by transfering its oxygen to them. Following are the oxidising agents for the given reactions:
(a) Pb3O4 as it is oxidising HCl to H2O
(b) O2 as it is oxidising Mg to MgO
(c) CuSO4 as it is oxidising Zn to ZnSO4
(d) V2O5 as it is oxidising Ca to CaO
(e) H2O as it is oxidising Fe to Fe3O4
(f) CuO as it is oxidising H2 to H2O