Out of benzene, m–dinitrobenzene and toluene

The ease of nitration depends on the presence of electron density on the compound to form nitrates. Nitration reactions are examples of electrophilic substitution reactions where an electron-ich species is attacked by a nitronium ion $\left(\mathrm{NO}_{2}-\right)$.

Now, $\mathrm{CH}_{3}-$ group is electron donating and $\mathrm{NO}_{2}-$ is electron withdrawing. Therefore, toluene will have the maximum electron density among the three compounds followed by benzene. On the other hand, m- Dinitrobenzene will have the least electron density. Hence, it will undergo nitration with difficulty. Hence, the increasing order of nitration is as follows: