Chapter 16 - Electric Potential, Energy, and Capacitance - Learning Path Questions and Exercises - Multiple Choice Questions - Page 589: 13

(e) either (c) or (d) could be correct

Electric field is always perpendicular to equipotential surface so here they will horizontal as vertical planes are equipotential surface. However, we cannot tell about the direction as we do not know whether the potential is decreasing to left or to the right.