(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 $S p^2$ 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.