This question is written by a master of deception and distraction.
It's full of so many red herrings that it's tough to walk past it too closely.
In order to answer this question correctly, you don't need to know
the magnitude of the test charge, the distance it travels, or the field
strength between the plates. The whole question lies in that magic
word "equipotential".
An 'equipotential' line is a path along which the electrical potential
is the same at every point. It's analogous to a 'contour line' on a
topographic map, or an isobar on a weather map.
A charge moving along such a line uses no energy to travel that path,
and it encounters no change in voltage anywhere in its travels.
