I didn't guess Czechoslovakia because I thought it was part of the Austro-Hungarian Empire. After the quiz I checked a map and it looks like I'm right.
San Marino - 20 volunteers joined on the Italian (Allied) side and it is claimed that San Marino was at war with Austria–Hungary, but it never entered the war. -Wikipedia
The quiz is supposedly for countries that fought in WW1 - but NONE of the Central and South American countries mentioned did any fighting - and nor did Liberia, China or Siam. They might have declared war but they did not fight. But Australia, New Zealand, Canada, South Africa and Newfoundland aren't mentioned...