Why carbonyl groups are strong benzene deactivating group for electrophillic aromatic substitution?
A carbonyl group such as an aldehyde is a deactivating group as it withdraws electron density by both the inductive effect (through the sigma bonds) and the resonance effect which involves pi systems.

Here are three resonance forms which show how the aldehyde group takes electron density from the benzene ring in benzaldehyde. There are more resonance forms and some different but related ones account for the reduction in the stability of the cationic intermidate in an aromatic substitution reaction.