One event that played a crucial role in transforming the U.S. into an industrial powerhouse would be
"a. the Civil War. " since this led to a transition from agriculture to industry (to some extent) in the South.
Step-by-step explanation:
The American Civil War was a war persisted in the United States from 1861 to 1865. As a consequence of the long-standing dispute over captivity, war broke out in April 1861, when Federal forces attacked Fort Sumter in South Carolina, shortly after U.S. President Abraham Lincoln was initiated.