Final answer:
The electric field is uniform for both an infinite charged plate and for two infinite, charged parallel plates, with the field for the parallel plates being effectively doubled between them. Therefore, the correct option is B.
Step-by-step explanation:
The electric field of an infinite charged plate is uniform and perpendicular to the surface of the plate, as the charges distribute themselves evenly across the surface. When considering two infinite, charged parallel plates with opposite charges, the electric field is also uniform and perpendicular to the plates, but in this case, it's also effectively doubled in strength between the plates compared to a single infinite sheet of charge, because the fields from the two plates add together. The presence of uniform electric fields in these systems can be understood from the properties of conductors in electrostatic equilibrium, where the excess charges spread out uniformly. Essential Knowledge indicates that the electric field in such a parallel plate configuration is constant in both magnitude and direction at points far from the edges of the plates.
The correct answer to the student's question is that the electric field is uniform for both cases, which corresponds to option B: Electric field is uniform for both cases.