The correct answer is C) existing states fought over whether or not slavery should be allowed in new territories.
Slavery increased in the Southern states but the Northern states took steps to stop it. The answer that best explains how this division affected the westward expansion is "existing states fought over whether or not slavery should be allowed in new territories."
The nation was totally divided into two portions due to slavery. The Northern states that supported abolition and the Southern states that supported slavery. The Confederate states depended so much on slavery to work in agriculture and growing crops in the Southern plantations. Both sides engaged in the Civil War due to these differences. And after the end of it, the states were still having debates over the abolition or not of slavery in the westward expansion.