B

Step 1: When considering the behavior of two parallel beams of electrons, we need to recognize that electrons carry a negative charge.
Step 2: According to the principle of electromagnetism, like charges repel each other. Since both beams consist of negatively charged electrons moving in the same direction, they will exert a repulsive force on each other.
Step 3: The direction of this repulsive force is in the plane of the paper because the magnetic field due to each beam of electrons will create forces that are perpendicular to the direction of the current (in this case, the motion of the electrons).
Step 4: Therefore, the mutual force produced by the two beams of electrons is one of repulsion in the plane of the paper.
Therefore, the correct option is B.