This is very difficult to answer precisely because the terms are not well-defined.  Where, exactly, do we say the Eastern Front was?  What do we do about countries like the Austro-Hungarian Empire that could be both near a front and between the fronts?  Do we look at the absolute number of people who were killed or at the percentage of the population?  In other words, I hope that you are allowed to explain your answer rather than just picking one of the options.

I agree with you that C is clearly incorrect.  I think that you would probably say that D is incorrect as well because countries far from the front (US, Australia, etc) were not as likely to have large numbers of deaths.  This means that there is some correlation between distance from the front and casualties.

However, there is no good way to distinguish between A and B.  For example, France was very near the Western Front and lost about 4.3% of its population in the war.  On the other hand, Serbia was not on the Eastern Front but it lost 16% of its population.  There is not a clear correlation here between distance from the front and deaths.

What I would do is to list the countries in order of how many deaths they suffered.  You'll have to decide between doing this in terms of absolute numbers or a percent of the population.  Then you should designate each country as on the front, between the fronts, or far away.  Look at the numbers and make your own conclusion that you feel you can support.  My own inclination is to say that within Europe there was no relation between distance from the front and deaths.  But that is not really one of the options you're give.