Question

why was it threatening to the united states that one or more of the states could declare that they would disregard federal legislation?
o because it would give rich states more power than poor states
o because it would make one state superior to another
o because such acts could dissolve the union itself
o because it would create the need for greater enforcement authority

Explanation:

The United States is a federal union. If states can disregard federal legislation, it undermines the unity and authority of the federal system, potentially leading to the dissolution of the Union. The balance of power and the very existence of the nation - state as a unified entity are at stake.

Answer:

because such acts could dissolve the Union itself