I agree. Totally weird.

Now they are accepting it.

First: watch using the ` in place of '. I think on a Portuguese keyboard it's on the top left where we usually have the for "~" or "`" on English keyboards.

As far as "which" vs "what": in this sentence (in English) you'd probably only use "which" when given a choice of multiple population numbers. i.e. "Given 70, 70 thousand, or 70 million; which is the closest population of France?" But if you had no specific choices (it could be any number), you just ask what it is like the question given.

Probably because the people who inputted it speak Portuguese as a native language and make mistakes. Just flag it and tell them what's wrong so they can fix it the next time they make it to this exercise.