a
In the given substituted benzene rings, the substitutents methoxy \((- OCH_3)\) and amino \((-NH_2)\) are strongly activating groups while methyl \((- CH_3)\) is weakly activating and chloro \((- Cl) \) is a deactivating group towards electrophilic aromatic substituation reaction. Since among methyl and methoxy group, methoxy group is more reactive than methyl group, \(( c)\) is more reactive than \((d)\). Although amino group is strongly activating group, it gets protonated in presence of acid to form anilinium ion \(\left( { - \mathop N\limits^ +  {H_3}} \right)\) which is strongly deactivating. Hence, \((a)\) is less reactive than \((c)\) and \((d)\) . Chloro group is also deactivating group but  less deactivating than \(\left( { - \mathop N\limits^ +  {H_3}} \right)\). Thus order is \((a) < (b) < (d) < (c)\). Note: The activating groups increases the electron density on benzene ring and increases the rate of electrophilic aromatic substitution reaction. The deactivating groups decreases the electron density on benzene ring and decreases the rate of electrophilic aromatic substitution reaction.