Q.In which of the following molecules, all atoms are co-planar

Structure of Benzene (Kekulé and Resonance)

Benzene's molecular formula, C6H6, signals a highly unsaturated compound, but its chemistry (no reaction with bromine/CCl4, acidified KMnO4, or water+acid the way an alkene or alkyne would) ruled out a simple open-chain structure. Two lines of evidence established instead that benzene is CYCLIC: it gives only ONE monosubstitution product with Br2/AlCl3 (possible only if all six H's, and hence all six ring carbons, are equivalent), and it adds exactly THREE moles of H2 over Raney nickel to give cyclohexane (confirming both the six-membered ring and three double-bond-equivalents' worth of unsaturation).

KEKULÉ (1865) proposed a cyclic, planar hexagon with ALTERNATING single and double bonds. This faced two objections: it predicts TWO distinct ortho-disubstituted products (one substituent pair flanking a double bond, the other flanking a single bond) where only ONE is ever observed; and it cannot explain why benzene, despite three formal double bonds, resists the addition reactions typical of alkenes. Kekulé's own fix -- proposing a rapid equilibrium between the two alternating-bond arrangements -- was later superseded by the modern RESONANCE description: the two Kekulé structures are not real, interconverting molecules but merely two CONTRIBUTING structures on paper, and benzene's true structure is a single, static RESONANCE HYBRID of both, with the pi electrons genuinely delocalised rather than shuttling back and forth. …