The most appropriate conclusion from the options provided regarding special interests and the Civil War is:
D) Special interests were among the causes of the Civil War.
Special interests, such as those related to the economy (e.g., the cotton industry, agriculture in the South, and industrialization in the North), played significant roles in the tensions that led to the Civil War. Issues like states' rights, slavery, and economic disparities fueled by these interests contributed to the divide between the North and South. While some groups might have profited from the war (making option C also relevant), the overarching influence of special interests as a factor leading to the conflict aligns best with option D.