(i) As the constant electric field in the Z-direction, the equipotential surfaces are normal to the field, i.e. in XY-plane. The equipotential surfaces are equidistant from each other.
(ii) As, the electric field increases in the direction of Z-axis, the equipotential surface is normal to Z-axis, i.e. in XY-plane and they become closer and closer as the field increases.
(iii) As a single positive charge is placed at origin, the equipotential surfaces are concentric spheres with origin at centre.