The best description of the conditions faced by soldiers during the Civil War among the provided options is: "Poor conditions were made worse in that most soldiers were young men under the age of 21."
While improved military technology did lead to higher casualty rates, not all soldiers died instantly from wounds. Additionally, food scarcity and unsuitable clothing were significant issues for soldiers on both sides, but the statement about Southern cotton does not accurately reflect the overall conditions. Lastly, Prisoners of War were often subjected to harsh conditions rather than being treated well.