The angle between electric lines of force and equipotential surface is:
A. ${0^ \circ }$
B. ${180^ \circ }$
C. ${90^ \circ }$
D. ${45^ \circ }$

Hint:Electric field lines or electric lines of force are ways of visualizing electric fields. Electric lines of force are defined as a way or path, it can be either straight or curved such that the tangent gives the direction of the intensity of the electric field at that time at any point.

Complete answer:
We know that an electric field line is described as the physical field which adjoins each charge and exerts force on all other charges within the field, it can either be an attracting force or a repelling force.
The equipotential surface is defined as the surface which is a locus of all the points that are at the same potential. No work is needed to move a charge from one point to another on an equipotential surface. In other words, we can say that any surface with the same electric potential at every point is known as an equipotential surface.
Some properties of equipotential surfaces are:
1) On an equipotential surface the electric field is always perpendicular.
2) Intersection of two equipotential surfaces is not possible.
3) The equipotential surfaces are concentric spherical shells, for a point charge.
Now, according to the given question, we know that when the potential becomes constant, the negative potential gradient also becomes zero, which is necessary for the need of the electric field to be always normal with the surface.
Thus, the angle between electric lines of force and equipotential surface is ${90^ \circ }$.

Hence, option (C) is the correct answer.

Note:
In order to find the solution of these kind of question students are needed to be aware of some of the properties of electric field lines such as,
1) Two field lines never intersect one another.
2) Electrical field lines are perpendicular to the surface of the charge.
3) Both the magnitude of the charge and the number of field lines are proportional to each other.