Final answer:
W.E.B. Du Bois identified racism as the primary cause of the social problems affecting African Americans in the late 19th century, which he examined through sociological studies and his work with the NAACP.
Step-by-step explanation:
According to W.E.B. Du Bois, the underlying cause of the social problems that affected African Americans at the end of the 19th century was primarily due to racism. Du Bois dedicated his life to the sociological study of Black life in America, uncovering the pervasive nature of racism and its effects on African Americans, including socioeconomic disenfranchisement and self-perception. His influential works, such as The Souls of Black Folk, analyze the notion of the color line and how it demarcates the racial divide affecting all aspects of life for Black Americans. Through his work with the NAACP and other civil rights endeavors, Du Bois sought to address and dismantle the systemic barriers that perpetuated inequality.