(a) Resonance structures of phenol are Phenol is a planar molecule. The p-orbitals on each carbon atom lie perpendicular to the plane of the molecule, the oxygen atom lying in its p-orbital is also perpendicular to the molecule. Consequently the p-orbital overlap, between O and C takes place and the resonance structures arise. (b) $\mathrm{–NO_2}$ is an electron withdrawing group. As a result, it polarises the benzene molecule and resonance occurs as shown : (c) $\mathrm{CH_3CH=CHCHO}$ is an α,β-unsaturated aldehyde where the C = C gets polarised due to the neighbouring electron withdrawing – CHO group. (d) Oxygen being highly electronegative pulls the π-electron cloud of the C=O towards itself thereby leaving the carbonyl carbon with a positive charge. In order to compensate this electron deficiency, the electron cloud from the ring moves towards it and the resonance structures arise.