Answer:
Higher the electron density in the benzene ring, more reactive is the aromatic compound towards electrophilic substitution reaction. Now due to the presence of a lone pair of electrons on the N-atom which is can directly donate to the benzene ring, \[N{{(C{{H}_{3}})}_{2}}\] is a much stronger electron donating group than \[C{{H}_{3}}\] group. The remaining two groups contain a positive charge on the N-atom and hence act as electron-withdrawing groups. But in \[{{(C{{H}_{3}})}_{3}}\overset{+}{\mathop{N}}\,-\]group, the +vely charged N is directly attached to the benzene ring, therefore, its electron-withdrawing ability is much stronger than \[-C{{H}_{2}}\overset{+}{\mathop{N}}\,{{(C{{H}_{3}})}_{2}}\]. From the above discussion, it follows that the electron density in the benzene ring increases in the order :
Therefore, their reactivity towards electrophilic substitution reactions also increases in the same order.
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