If a charged particle goes unaccelerated in a region containing electric and magnetic fields.

The charged particle must not accelerate in the region. If both the field are counter balance, then the particle won’t accelerate. If Electric force is acting along the direction of magnetic force and both are same then the particle will not effect by these fields. So particle won’t accelerate. And if the electric field is parallel to the velocity of a particle then it will definitely accelerate. So electric field must be perpendicular to the direction of the velocity. So options A and B are correct.