Which of the following is the most reactive towards attack by an electrophile ?

Explanation

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. For more details, consult section 16 (Chemical properties of phenols).