C. This matter of geography affected the spies who worked behind enemy lines.
In The Dark Game, the second paragraph discusses the challenges spies faced due to the geographical layout of the war. The Union and Confederate forces were often fighting in unfamiliar territories, and this geography had a direct impact on the spies who had to navigate these areas. It made their jobs more difficult and dangerous as they worked behind enemy lines.
While this is a factual statement about the geography of the war, it doesn't address the central idea of the second paragraph, which is about the impact of geography on spies.
This focuses on the Union soldiers but does not specifically address the role of spies in the context of geography.
This talks about Confederate spies, but the central idea of the paragraph revolves more around the challenges spies faced due to geography, not just the safety of their locations.
Therefore, C. This matter of geography affected the spies who worked behind enemy lines best reflects the central idea of the second paragraph.