It would be helpful if you could explain in the answers why the country is the odd one out. Honestly there could be more than one answer for any of these, depending on the criteria.
Well, there are other answers that could work. For example, in the first one, Malta is the only country with fewer than 1 million inhabitants. And in the third one, South Sudan is the only country that became independent after 2000.
Australia, being a continental landmass, is not an island. If you were to count Australia as an island, you would also have to count Antarctica, America and the Afro-Eurasian landmass as islands.
Second one: only country not in europe
Third one: only country that is an island
Fourth one: only country that starts with a vowel
I hope this is what you were looking for!
This could -- only if you're willing to delete this quiz, and only use 8 Countries -- maybe work as a Multiple Choice Quiz.
And please put the reasons in the quiz, not as a reply.