Explanation:

- The correct option is: Option 3: Blokhus (Denmark).

- 21.16 m Sandcastle: The sandcastle that holds the Guinness World Record for being the tallest was built in Blokhus, Denmark. It was completed in 2021 and reaches a stunning height of 21.16 meters (69.4 feet).

- Option 1: Hague (Netherlands): Known for the International Court of Justice, it has beautiful beaches but not a record-breaking sandcastle.

- Option 2: Warsaw (Poland): Poland's capital is rich in history and culture but is not known for its sand sculpture records.

- Option 4: Stockholm (Sweden): The Swedish capital is famous for its islands and cultural history, not large sand structures.

- Option 5: Oslo (Norway): Known for its museums and green spaces, not for large sandcastle constructions.