An electrophile will always prefer to attack centres of high electron density in the ring. Both nitro `(-NO_(2))` and chloro `(-Cl)` groups are electron withdrawing groups. In benzyl alcohol (III), the `OH` group is not directly attached to the ring and is not in a position to activate it. Phenol (I) is the most reactive because phenolic group activates the ring towards electrophilic attack both at the ortho and para positions.
