It's based on probability. The mathematical answer is always 4, therefore the country's name will always start with "D".
For at least 99 per cent of the population, the only "D" country they can think of is Denmark. The only other "D' countries (in English) are Djibouti, Dominica and Dominican Republic. They don't come to mind nearly as easily as Denmark.
Likewise, most people will say "kangaroo" when asked to name an animal whose name begins with "k". Not many will say "koala" or "kookaburra" (non-Australians anyway). The same applies for a colour beginning with "o". Not many will say "ochre".
Deutschland is cheating! It;s not English!
