The South was the one invaded by the North, since Abraham Lincoln wanted control over the South instead of recognizing it as a State. By considering the states of the South rebellious, they were the ones that needed to be forcefully controlled, therefore it was them the ones to be invaded. By being invaded, you have the advantage of knowing the territory.
Just by common sense we can rule out both C and B because neither of those make sense for this case. High population really doesn’t matter. So the most logical answer is D. Mostly Defending Familiar Territory
