The order of decreasing reactivity towards an electrophilic reagent, for the following, would be:
(a) Benzene
(b) Toluene
(c) Chlorobenzene
(d) Phenol
d > b > a > c
We are talking about the reactivity of an aromatic electrophilic substitution reaction based on the nature of substituents (if any) present on the benzene ring. Groups like −OH and −NH2 are strongly activating in nature. It means that such groups can donate an electron pair via conjugation and increase the electronic charge density of the ring, thereby making the whole process easier. Alkyl groups are mildly activating in nature via hyperconjugation or no-bond resonance. Chlorobenzene is the least reactive for an electrophilic aromatic substitution because the chlorine atom is overall electron-withdrawing via inductive effect. Its electron-withdrawing inductive effect is more pronounced than its electron-donating conjugation.
The correct order is d > b > a > c