The question assumes too much. Or maybe it is just phrased inartfully, as one of the listed options is very much not a "country."
That said, I didn't vote for the European Union - and I wouldn't even classify it as a plausible candidate. Nevertheless, I doubt that the most powerful political entity in 2100 will be anything that we would recognize as a country.
I don't know. Had you asked this question in 1900 I think the correct answers in 1950 or 2000 would have been recognizable political entities.
Poll
