Define an equipotential surface. Draw equipotential surfaces:

(i) in the case of a single point charge and in a constant electric field in Z-direction.

(ii) Why the equipotential surfaces about a single charge are not equidistant?

(iii) Can electric field exist tangential to an equipotential surface? Give reason.

ii) The equipotential surface about a single charge is not equidistant because V is inversely proportional to r.
Also, the equipotential surfaces about a single charge are not equidistant because electric field due to a  single charge is not constant.
iii) Electric field cannot exist tangential to an equipotential surface because if the field lines are tangential, work will be done in moving a charge on the surface, which is against the theory of equipotential surface.