Surely Rotterdam, the second-largest city in the Netherlands is the answer for "R"? Also, why have you included Den Haag under "D"? Indexes never consider the definite article - or do you think that the Netherlands begins with a T? Den Haag obviously starts with an "H".
Thank you for your input! I did not realize I had to resubmit and not just save again for my changes to take effect (which would have solved the "Rotterdam" issue a lot quicker). As for the articles, I did not really know what to do with them when I first started making the quizzes, but have now changed it as you suggested.
I agree that the Hague should be placed under H, but that's because the name in English is the Hague (where you should ignore the definite article). However, if the definite article is part of the name borrowed from the original language, then I believe it should be included in the full name. El Salvador is always included under E in the JetPunk quizzes I have taken.