Basics of Organic Chemistry

For compound in option (A): Only two of the conformers (cisiod and transoid) have all the atom in the same plane.
For compound in option (B): The terminal hydrogen of allene will be perpendicular to each other plane.

For compound in option (C): All the atoms are in one plane in all the possible conformations. There is no atom on oxygen.

CH2

sp2

=

C

sp

=O