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