Answer
We can rank the six choices according to the potential difference set up across the solid:
$(+z = -z) \gt (+y = -y) \gt (+x = -x)$
Work Step by Step
When the solid moves in the +x direction, there is no force on the electrons and the potential difference is zero.
When the solid moves in the -x direction, there is no force on the electrons and the potential difference is zero.
When the solid moves in the +y direction, there is a force on the electrons that pushes them to the front face. The potential difference is $V = vBd$
When the solid moves in the -y direction, there is a force on the electrons that pushes them to the back face. The potential difference is $V = vBd$
When the solid moves in the +z direction, there is a force on the electrons that pushes them to the bottom face. The potential difference is $V = 2vBd$
When the solid moves in the -z direction, there is a force on the electrons that pushes them to the top face. The potential difference is $V = 2vBd$
We can rank the six choices according to the potential difference set up across the solid:
$(+z = -z) \gt (+y = -y) \gt (+x = -x)$
