respreng is right. Another way to remember this is that "qué? is looking for a definition, i.e. what does "the population of France mean?" A possible answer would be: it means the number of people who live in France. So you see that qué? is not what you need here, but rather cuál?
But there is only one definite number for the current population of France.
Now (if I'm interpreting rspreng's explanation correctly) if someone asked "What do you want?" Then you'd probably use "Qué". Since there are any number of things you could want and there's more choice and variability in what you could say to answer.
Because in context in this question, "Cuál" means "what". "What is the population of France?" makes more sense than "Which is the population of France", right? At least that's how I visualize it unless I know further. This creates some confusions when considering a scenario in which we are given a set of populations of different countries and we want to ask which one of them would be the population of France, but I am guessing there would be some rule in Spanish that takes care of that.
There are certainly enough examples of such strange questions being counted right, that one would not question such a translation. I have always learned that "cual" means "which." "Which" should be listed as another translation. Unless there is an actual context, it could mean exactly what it says.
It's just like one of those words in English that can be used in multiple ways depending on the context. For example, we can use "you" to refer to either one person or multiple people, or we can use "They" for either Masculine and Feminine or both. Or "right" can either mean "'right' or wrong" or "left and 'right'".