Answer
The direction of the electric field must point from plate A to plate B.
Work Step by Step
If the electric field slows the electron as the electron travels toward plate B, then the force on the electron due to the electric field must be directed away from plate B.
We can write the equation for the force on a charge due to an electric field:
$F = q~E$
An electron has a negative charge.
Since the charge $q$ is negative in the case of an electron, the force vector and the electric field vector must point in opposite directions.
Therefore, the direction of the electric field must point from plate A to plate B.
