To solve this problem, we need to understand the relationship between electric field lines and equipotential surfaces.$\\$• Electric field lines represent the direction of the electric field at any point in space.$\\$• Equipotential surfaces are surfaces where the electric potential is constant.$\\$The key property of equipotential surfaces is that no work is done when moving a charge along the surface,$\\$which implies that the electric field is always perpendicular to the equipotential surface.$\\$Let's analyze why the angle is $90^\circ: \\$• If the electric field had a component parallel to the equipotential surface,$\\$ it would do work on a charge moving along the surface, which contradicts the definition of an equipotential surface.$\\$• Therefore, the electric field must be entirely perpendicular to the equipotential surface.$\\$Thus, the angle between the electric lines of force and the equipotential surface is $90^\circ. \\$This corresponds to Option 3.