(a) The equipotential surfaces for a constant electric field in the z-direction are the equidistant planes parallel to the x-y plane.

(b) The equipotential surfaces for a field that uniformly increases in magnitude but remains in a constant in z direction are the planes parallel to the x-y plane and this field increases when the planes get closer.

For both cases A and B the Equipotential Surface is same as below

(c) The equipotential surfaces for a single positive charge at the origin are the concentric spheres centered at the origin.

D. The equipotential surface for a uniform grid consisting of long equally spaced parallel charged wires in a plane, near the grid is shaped as a periodically alternating curve. At a larger distance from the grid, the surface becomes parallel to the grid.