(a) In Chlorobenzene structure, chlorine is attached with a benzene ring through single bond and it is highly electronegative in nature. Thats why any electronegative/electrophilic agent can not replace it.
(b) There is delocalization of lone pair of electron present on chlorine atom. Carbon atom of benzene ring is Sp2 hybridized. Its structure is stabilized by resonance, so replacement of chlorine atom with other nucleophilic substance will required lots of energy which is not favourable.