D

Step 1: Review the definitions of electrostatic force and fields.
Step 2: Electrostatic force is indeed a conservative force, meaning that the work done in moving a charge between two points is independent of the path taken.
Step 3: The potential at a point is defined as the work done per unit charge in bringing a charge from infinity to that point, which is true.
Step 4: An equipotential surface is defined as a surface where the potential is constant, which also holds valid.
Step 5: Now, for Option D, while it's true that inside a conductor in electrostatic equilibrium, the electric field is indeed zero, there can be points within certain other materials where the electric field can be non-zero. However, within the confines of ideal conductors, the statement should be interpreted correctly in the context given.
Conclusion: As per the common understanding and simplifications often taken in electrostatics, stating that 'inside a conductor the electrostatic field is zero' excludes exceptional cases where fields may arise under non-ideal conditions. Hence, it can lead to misleading interpretations.
Therefore, the statement that is not true is option D.