Final answer:
The term for the migration of Black Americans from the South to Northern cities is the Great Migration, involving around 1.5 million people between 1910 and 1930, which had a significant impact on U.S. demographics. Option a is the correct answer.
Step-by-step explanation:
The term that describes the internal migration of 400,000 Black Americans from the South to Northern cities in search of work is known as the Great Migration.
During this period, approximately 1.5 million African Americans left the American South between 1910 and 1930 to take advantage of employment opportunities in Northern factories, especially during World War I.
This mass relocation had a lasting impact on the demographic landscape of the United States, as it led to a significant redistribution of the African American population, making industrial cities in the North and the Midwest major centers for African American culture and community life.