Final answer:
The question about the minimum distance between fence posts lacks context for a precise answer. In physics, electric-field lines are proportional to charge; a +20 nC charge should have four times the number of lines as a +5 nC charge, thus 20 field lines.
Step-by-step explanation:
The minimum distance between two fence posts can vary significantly depending on the specific requirements or regulations in different regions or for different applications. Therefore, without additional information or context, it's not possible to provide a definitive answer to the question about the minimum distance between fence posts. However, we can address the physics-related question that revolves around electric field lines and their quantity emerging from a point charge.
Electric field lines visually represent the field around a charged particle, and their number is typically proportional to the quantity of charge. In the scenario where five electric-field lines come out of a +5 nC charge, you would expect the number of lines to increase proportionally with the amount of charge. Consequently, a +20 nC charge, having four times the charge of a +5 nC charge, should logically have four times the number of electric-field lines emanating from it.
Thus, the answer would be d. 20 field lines, indicating a direct relationship between the number of field lines and the amount of charge.