Which of the following statements about australia is true? it is the only former european colony in the southern hemisphere. it is the only country that is also a continent. it has the highest average elevation of any country in the world. it is the only country with a mostly desert climate.

Let's look at the first. 

Australia was indeed a European colony, but it was not the only one. I've attached an image of all of the places Great Britain has invaded. 
The second statement is true. No other country takes up an entire continent. 
The third statement is false. The country with the highest average elevation is Bhutan, at 10,760 feet above sea level. 
The fourth and final statement is also false. Take Egypt, for example. It is almost completely desert. 

Hope this helps!

D. continental rift