Describe schematically the equipotential surfaces corresponding to
$(a)$ a constant electric field in the $z$-direction,
$(b)$ a field that uniformly increases in magnitude but remains in a constant (say,$z$) direction,
$(c)$ a single positive charge at the origin,and
$(d)$ a uniform grid consisting of long equally spaced parallel charged wires in a plane.

(N/A) Equidistant planes parallel to the $x-y$ plane are the equipotential surfaces.
$(b)$ Planes parallel to the $x-y$ plane are the equipotential surfaces,but the spacing between them decreases as the field magnitude increases.
$(c)$ Concentric spheres centered at the origin are the equipotential surfaces.
$(d)$ Near the grid,the equipotential surfaces have a periodically varying shape. At large distances from the grid,these surfaces gradually become planes parallel to the grid.