Answer
The bottom plate is at a higher electric potential.
Work Step by Step
Just before the electron enters the region between the bottom pair of plates, the force on the electron due to the magnetic field is directed upward.
The electron goes straight through the bottom pair of plates.
Therefore, the force on the electron due to the bottom pair of plates must be directed downward. Then the bottom plate must be at a higher electric potential since the charge on an electron is negative.
