The response that best shows how the issue of states' rights contributed to the onset of the Civil War is:
Many Southerners believed the rights of their states were threatened by Northern abolitionists.
This statement highlights the Southern perspective that the abolitionist movement and the policies coming from the North were encroaching on their rights as states, particularly their right to maintain the institution of slavery. This belief in the primacy of states' rights was a significant factor in the Southern states' decision to secede from the Union, ultimately leading to the Civil War.