a
\(- Cl\) and \(-CH_3\) groups are \(o\) and \(p\) directing. They are electron releasing due to \(+ E\) and \(+ M\) effects. Further since such groups increase electron density in the nucleus, they facilitate further electrophilic substitution and hence known as activating group. The activating effect of these groups is in order of \(- CH_3 > -X\) but chlorine exceptionally deactive the ring due to strong \(- I\) effect. Hence, it is difticult to carry out substitution in chlorobenzene than in benzene . Further \(- NO_2\) is a deactivating group hence deactivates the benzene nucleus, i.e. hinder the further substitution. Thus nitrobenzene undergo electrophilic substitution with a great difficulty hence the correct order will be