Question

the order of decreasing reactivity towards electrophilic reagent :
benzene,toluene,chlorobenzene,phenol

Answer 1

Benzene having any activating group i.e, OH, R etc., undergoes electrophilic substitution very easily as compared to benzene itself. Thus toluene (C6H5CH3), phenol (C6H5OH) undergo electrophilic substitution very readily than benzene.Chlorine with +E and +M effect deactivating the ring due to strong -I effect. So, it is difficult to carry out the substitution  in chlorobenzene than in benzene, so correct order is 

Phenol > Toluene > Benzene > Chlorobenzene

Benzene having any activating group i.e, OH, R etc., undergoes electrophilic substitution very easily as compared to benzene itself. Thus toluene (C6H5CH3), phenol (C6H5OH) undergo electrophilic substitution very readily than benzene.Chlorine with +E and +M effect deactivating the ring due to strong -I effect. So, it is difficult to carry out the substitution  in chlorobenzene than in benzene, so correct order is 

Phenol > Toluene > Benzene > Chlorobenzene