In which of the following, chlorine is least reactive ?

Chlorobenzene is less reactive towards nucleophilic substitution reaction due to resonance effect: The electron pair on chlorine atom is in conjugation with the electrons of the benzene ring which results in the following resonance structures:

This results in delocalization of the electrons of C - Cl bond and a partial double bond character develops in the bond, which makes it difficult for the Nucleophile to cleave the C - Cl bond.