Correct Answer - B::C
Along C-C single bond, numerous conformations are possible in 1,3-butadiene (a) . In some of these conformations , all the atoms may not lie in the same plane. In `CH_2=C=CH_2` (d), due to sp-hybridization of the central carbon, H-atoms on the terminal `CH_2` groups lie in perpendicular planes . However, in `CH_2=C=O` (c ), due to replacement of `CH_2` by O the two H-atoms on the `sp^2`-hybridized `CH_2` group lie in the same plane in which the two carbon and the oxygen atoms lie. In compound (b), due to linear structure of H-C`-=`C- portion of the molecule , the three H-atoms on -HC =`CH_2` portion always lie in the same plane. Thus, options (b) and (c ) are correct.
