The correct answer is - It is the only country that is also a continent.
Australia is one of the biggest countries in the world. It is an island, a very big one, plus it has some minor islands in its territory, with the biggest of them being Tasmania. But Australia is not just a huge country, it also a country that is a continent itself, and it makes it unique, as there isn't another country in the world that can say the same thing about its territory. As a continent, Australia ranks on the last 7th place by size, and as a country it ranks as the 6th biggest in the world.