Questions On Turning Thirty-One, part I by KlionskyG

Question 9

You would dial +31 to reach someone in what country, the most densely populated European country larger than 200 square miles?

For what it's worth, this country is also mentioned in Article 31 of the Treaty of Versailles and is part of Region 31 of the Sweet Adelines International (along with England, Scotland, and Wales).

The Netherlands