Amongst the given options, the compound(s) in which all the atoms are in one plane in all the possible conformations (if any), is (are)
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Planar molecules have all atoms lying in the same plane. This occurs with sp2 or sp hybridization and no steric hindrance.
Option 1: The central carbon is sp hybridized, linear. The terminal CH2 groups are sp2 hybridized. The hydrogens can rotate but the entire molecule remains planar in all conformations.
Option 2: The ring is aromatic and planar. The methyl group (-CH3) is sp3 hybridized; its hydrogens are not coplanar with the ring due to tetrahedral geometry, so not all atoms are in one plane.
Option 3: H2C=C=CH2 (allene). The central carbon is sp hybridized, making the terminal CH2 groups perpendicular. Rotation is restricted; it is not planar in all conformations.
Option 4: H2C=C=O. The carbon is sp2 hybridized, and the molecule is rigid and planar.
Final Answer: Options 1 and 4 have all atoms in one plane in all possible conformations.