Would it be possible to identify the incorrect choice when the answer appears? For example, could you include "The other flag is Saint Kitts and Nevis." in the answer to question 15? This is a great quiz!
I would suggest Luxembourg as the flag next to the Netherlands, since it is 99% guessed correctly. The coat of arms is a dead give away (maybe not everyone will be able to correctly identify what that flag is, Paraguay (34% on flags of the world) but they will know the Netherlands does not have a coat of arms. With just different colors it is more of a challenge.
Nice quiz!
https://www.jetpunk.com/user-quizzes/1744710/banderas-de-paises-similares-opcion-multiple
Serbia/Croatia - stuff on top of the shield
Serbia- squished, like its geographical position
Slovenia- long like its outline
It's easier when seeing all 4 together.
Serbia : Eagle
Slovakia : Cross
Slovenia : Mountain