Final answer:
The US Civil War significantly increased the power of the federal government while decreasing states' rights, especially after the Union victory and the imposition of conditions on Confederate states' readmittance to the Union.
Step-by-step explanation:
The correct answer to the question is B) US Civil War. The Civil War had a profound impact on the balance of power between the federal government and the states. The Union victory firmly established the primacy of the national government, ending the right of states to secede and challenging legitimate national laws. Subsequent actions, such as the Emancipation Proclamation and the imposition of conditions on the readmittance of former Confederate states—including the ratification of the Fourteenth and Fifteenth Amendments—significantly increased federal power and decreased states rights.