A mixture of O2 and N2
Moist O2
Dry O2
O2 in the presence of aqueous NaOH
P4 + 3O2 → P4O6
White phosphorus on reaction with a limited supply of oxygen gives lower oxide, P4O6. Therefore, air (O2 + N2) is a good source for the controlled supply of oxygen and the best choice for controlled oxidation of white phosphorus into lower oxide. Hence, option B is correct.