The answer to this question is Indonesia. You can determine this by using lines of longitude on a map.
If you look on a map that has lines of longitude on it, you will see that the southernmost point of South America is somewhere around 68 degrees of longitude (West).
If you want to find what is exactly on the opposite side of the world, you must go around 180 degree because the whole world is 360 degrees. To go 180 degrees, you go back 68 degrees to get to zero and then another 112 degrees or so to get to a total of 180.
The southernmost land at that longitude (112 degrees East) is in Indonesia.
I should say that 115 E is the western most point of Australia so you would just miss Australia by a little bit.