Final answer:
Kentucky, Missouri, and Delaware were the border states that remained in the Union during the Civil War.
Step-by-step explanation:
The border states that remained in the Union during the American Civil War were Kentucky, Missouri, and Delaware. These states were geographically located between the North and the South and had populations with divided loyalties. Although they were officially part of the Union, they also had residents who supported the Confederacy.
Learn more about Union states during the Civil War