Which of the following is not a method of preparing oxygen?

Solution (By Examveda Team)

One of the two chemical methods starts from natural manganese dioxide and converts it using dinitrogen tetroxide and water to manganese nitrate solution. Evaporation of the water leaves the crystalline nitrate salt. At temperatures of 400°C, the salt decomposes, releasing N2O4 and leaving a residue of purified manganese dioxide.