(b) are perpendicular to the electric field
Work Step by Step
Consider an equipotential surface with electric field lines that are not perpendicular to the surface. These lines could then be resolved into two components. One perpendicular to the surface and one along the surface. The field line along the surface means that the charges would move along the surface in the direction of the field lines. So, there would be potential difference along the surface. But that is not possible since the surface was said to be equipotential with equal potential across the surface.