A. What is an electric field line? b. What are equipotential sufaces are they necessarily physical surffaces?
c. Are they necessarily physical surfaces?

Please see below as the answers are self-explanatory

Explanation:

a)

A electric field line is an imaginary line, which has the property that the electric field vector is tangent to it at any point. It starts from positive charges (since the electric field by convention it has the direction of the trajectory that would take a positive test charge, so it always goes away from positive charges) and ends in negative charges.

b)

Since the potential difference between two points represents the work per unit charge needed for a charge to move between these points, a equipotential surface is the one over which it is not needed to do work to move a charge from any point on the surface to any other point, which means that all points are at the same potential.

c)

Equipotential surfaces are not necessarily physical surfaces, they can be defined in vaccum for instance.As an example, any spherical surface concentric with a point charge, is an equipotential surface, and  it can be a real surface or a fictitious one.