Coastlines are surprisingly difficult to measure, and this is not merely a practical problem. Mathematicians call it the “coastline paradox”: the smaller the measuring units you use, the longer the coastline turns out to be. Measure in straight segments of ten kilometres and you get a modest figure. Follow every small inlet, every curve in the dune edge, and every tidal channel, and the number grows considerably. No two sources give exactly the same answer, and they all have a point.

For the Netherlands, it also matters what you count as “coastline.” Adding the shores of the Wadden Sea, the Zeeland estuaries, and the IJsselmeer produces a much higher figure than the approximately 350 kilometres of open North Sea coast. The figures in this question refer to that open sea coast, from the southwestern islands to the seaward side of the Wadden Islands.

What makes the Dutch coast particularly remarkable is that a large part of it is actively maintained. Dunes, dykes, and dams protect the land behind them, a considerable portion of which lies below sea level. Sand nourishment — where sand is deposited offshore to keep beaches and dunes at the right level — is not a one-off intervention but an ongoing process.

With estimation questions about coastal length, the trap lies in the definition. Answer A is too short for the full North Sea coast; the higher options are more in keeping with a count that includes all inland waters and island shores. Anyone who pauses to consider which definition the question is using will avoid being misled by the coastline paradox.