Equipotential lines are a way to visualize the distribution of electric potential in a given space. They are imaginary lines that connect points with the same electric potential. In other words, all the points along an equipotential line have the same potential difference from a reference point.

Electric field lines, on the other hand, represent the direction and strength of the electric field in a given space. The electric field at any point is defined as the force experienced by a positive test charge placed at that point.

Now, let's discuss the options provided and determine which one is true about equipotential lines.

a) Electric field lines are tangential to equipotential lines:
This statement is not true. Electric field lines and equipotential lines are perpendicular to each other. This is because the electric field is a vector quantity and its direction is always perpendicular to the equipotential lines. If the electric field lines were tangential to the equipotential lines, it would mean that the electric field is parallel to the equipotential lines, which is not the case.

b) Electric field lines are parallel to equipotential lines:
This statement is also not true. As mentioned earlier, the electric field lines are always perpendicular to the equipotential lines. If they were parallel, it would mean that the electric field is constant along the equipotential lines, which is not the case either.

c) Electric field lines are perpendicular to equipotential lines:
This statement is true. Electric field lines are always perpendicular to the equipotential lines. This is because the electric field is conservative, meaning that no work is done in moving a charge along an equipotential line. The direction of the electric field lines indicates the direction in which a positive test charge would move if placed at a particular point, and it is always perpendicular to the equipotential lines.

d) Electric field lines are opposite to equipotential lines:
This statement is not true. The term "opposite" is not well-defined in this context. Electric field lines and equipotential lines have different physical meanings and cannot be directly compared in terms of being opposite to each other.

In conclusion, the true statement about equipotential lines is that electric field lines are perpendicular to equipotential lines.