Explanation

(b) An equipotential surface is a surface with a constant value of potential at all points on the surface. For a uniform electric field, say, along the $x$-axis, the equipotential surfaces are planes normal to the $x$-axis, $i.e.,$
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Plane parallel to the $y - z$ plane on equipotential surface, potential at all points is constant, this means that on equipotential surface work done in moving a test charge from one point to other point is zero