The order of decreasing reactivity towards an electrophilic reagent for the following: (i) Benzene, (ii) Toluene, (iii) Chlorobenzene, and (iv) Phenol would be:

Reactivity in electrophilic substitution depends on the electron density of the ring.Phenol (iv): $-\text{OH}$ group has a strong $+M$ effect, highly activating.Toluene (ii): $-{\text{CH}}_3$ group has $+I$ and hyperconjugation effects, activating.Benzene (i): Standard.Chlorobenzene (iii): $-\text{Cl}$ has a $-I$ effect that outweighs its $+M$ effect, deactivating.Order: Phenol > Toluene > Benzene > Chlorobenzene.(iv) > (ii) > (i) > (iii).