A border drawn arbitrarily in 1890 is defended a century later as a national frontier worth dying for. What does that trajectory illustrate?

D. That lines become real by being administered

The line organises different schooling, taxation, conscription and law on each side, and after three generations the populations genuinely differ, the map preceded the reality and then produced it. Option 4 gets the mechanism wrong: it is administration doing the work, not duration.