Final answer:
The major effect of WWI on American society was the migration of African-Americans to the North, as they sought better opportunities and contributed to social and labor shifts within the country. Therefore correct option is C
Step-by-step explanation:
The major effect of World War I on American society in 1917-18 was not a reduction in income taxes, an increase in unemployment rates, or the entry of large numbers of women into the armed forces, but rather the migration of African-Americans to the North. This Great Migration saw hundreds of thousands of African-Americans move from Southern to Northern cities, seeking better economic opportunities and escaping Jim Crow laws. Additionally, the war effort created a demand for labor, resulting in over a million women entering the workforce, often in jobs that had been reserved for men. The war profoundly changed the social fabric of America, with lasting impacts on organized labor, race relations, and gender roles.