The reactivity of aryl halides towards nucleophilic substitution increases if any electron withdrawing group,such as NO2
is bonded to the benzene ring at the ortho and para position.
In chlorobenzene, no EWG present. So, it is least reactive.
In o-nitrochlorbenzene, one EWG is present in the ortho position.
In o,p-dinitrochlorobenzene, two nitro groups present. So, it is more reactive than o-nitrochlorbenzene.
In trinitrochlorobenzene, three EWG (NO2) present. So, it is most reactive among all the compounds.