Love the image for this one. Why does Slovakia/Slovenia question ask about Mediterranean and not just Adriatic? Also it should say "Which of these borders" not "Which of the borders."
Great quiz! Well-chosen pairs and distinguishing features. It's easy if you have some familiarity with the countries of the world, of course, as most regular Jetpunkers do; but these are in fact often confused by non-geography-enthusiasts, and there is space for quizzes of different difficulties here.
I might add Sweden and Switzerland.