The correct answer is D, because "their" signifie that they are having "their" conversation, and that they own the conversation. C is incorrect because "there" describes where, B is incorrect because "they" describe who, and A is incorrect because "they are" doesn't grammatically or sensibly fit in, so the answer is D.
One of the main reasons for the civil war was the differences with slaves about whether it was illegal or legal to have slaves. This could have been avoided if there was agreement on how slaves should be treated.
