The correct answer is B) Because such acts could dissolve the Union itself.
Why was it threatening to the United States that one or more of the states could declare that they would disregard federal legislation?
Answer: Because such acts could dissolve the Union itself.
If we are talking about the political situation in the times of the American Civil War, the Southern states continued to exert much pressure on the Union regarding the issue of slavery to the degree of threatening separation.
That is why during the Montgomery Convention of February 1861, the Confederate states were established with the agreement of South Carolina, Georgia, Louisiana, Florida, Alabama, Texas, and Mississippi.
Three months later, in July, four more states joined the Confederacy: North Carolina, Arkansas, Tennessee, and Virginia.