Which of these have all its atoms in one plane in all the possible conformations (if any):
While I was attempting this question, I got stuck in option B! I was sure about option A that since it has conformational change possible between C2-C3 bond, atoms will be present in more than one single plane.
Next, I am also convinced that option D can't be the answer as allenes are non planar and they have H-C-H planes perpendicular to one another. Then, for option C also, I know that since all the atoms are $sp^2$ hybridised, it has to be a planar straight molecule.
But, I am having a doubt in option B; how should we determine if the molecule is planar or not and whether all of its atoms are present in a single plane?