(a)
As the constant electric field is in z-direction therefore, the potential, in a direction perpendicular to the z-direction always remain constant.
Thus, the planes that are parallel to the x-y plane will be the equipotential surfaces.
(b)
In a direction perpendicular to the electric field, the potential remains constant irrespective of the magnitude of the field.
Thus, Thus, the planes that are parallel to the x-y plane will be the equipotential surfaces.
(c)
For a single positive charge that is located at the origin, the equipotential surfaces will be the concentric spheres and the centers of the spheres will be at the origin.
(d)
If a uniform grid consisting of long equally spaced parallel charged wires in a plane then the equipotential surface will be a periodically varying shape near the given grid and this varying shape will be gradually reached the shape of planes parallel to the grid at a larger distance.