$S_1\text{ and }{S}_2$ an, two equipotential surfaces on which the potentials are not equal 

All are conceptual points related to $EPS$, use the concepts ., 

i) If two equipotential surfaces intersect , then electric field at any point on the line of intersection will have two directions which is not possible .

iii) closer the equipotential surfaces, greater will be the field . 

iv) Electric field lines are always perpendicular to equipotential surfaces.

ii) For uniform electric field , equipotential surfaces are plane surfaces .