Which of the following is least reactive towards nucleophilic substitution with aqueous KOH?

Nitro group, being an electron withdrawing group decreases the electron density over the benzene ring.
Hence,presence of a nitro group at ortho or para position increases the reactivity of haloarenes towards nucleophlic substitution.

While the -OH group , being an electron releasing  group increases  the electron density over the benzene ring and makes less reactive in nucleophilic substitution reaction.

however C-Cl bond is stronger than C-Br bond 

Hence option-2 is correct.