The best name for the compound is DINITROGEN TRIOXIDE.
Each chemical compound is always represented by a chemical symbol, which shows the ratio at which each atom of the elements of the compound are combine together and this is often used in naming the compound. Looking at the compound given in the question, the compound is made up of two atoms of nitrogen and three atoms of oxygen and this fact was used in naming the compound. In naming chemical compounds, 'Di' stands for 2 while 'Tri' stands for 3. Since there are two nitrogen and three oxygen atoms in the compound, that was why it was named dinitrogen trioxide.