Final answer:
The electric potential at point B remains 200 V regardless of whether a 15nC charge or a 30nC charge is moved from point A to point B, as electric potential is determined by the electric field at point B and not by the charge itself.
Step-by-step explanation:
The question asks what the electric potential at point B would be if a 30nC charge, instead of a 15nC charge, was moved from point A to point B. The electric potential at a point is independent of the charge being moved and depends only on the electric field and the position in that field.
Since the potential at point B was determined to be 200 V when moving a 15nC charge, this potential will be the same if a 30nC charge is moved from A to B. Electric potential is a property of the electric field at point B itself and not of the charge that is moved. Therefore, the potential at B will remain 200 V regardless of whether the charge is 15nC or 30nC.