As you are aware that the electric field is always perpendicular to the surface of a conductor, and you are doubtful between B and C, I think your doubt must be whether the plates are conducting and if not, whether this condition applies also to non-conductors.
The question does not specify if the plates are conducting, so we should not necessarily assume that they are. It is possible to have charged insulating plates.
The condition applies to conductors because the charges in conductors are freely mobile. If the surface field has a tangential component then charge will flow and this component will be nullified. This is not the case for insulators : charge cannot flow freely. Therefore the condition does not apply for insulators/dielectrics.
So both B and C are possible. If the question-setter intended only one answer to be correct, then the question is flawed.