Final answer:
W. E. B. Du Bois is the individual who declared 'the problem of the twentieth century is the problem of the color line,' highlighting the racial divide in American society.
Step-by-step explanation:
The individual who said about American society, "the problem of the twentieth century is the problem of the color line," was W. E. B. Du Bois. This statement appears in the Forethought of his seminal work, The Souls of Black Folk, where Du Bois expresses the central theme of his book. The color line refers to the division between races, manifesting both as an invisible societal separation and, at times, a very physical barrier. Du Bois's work is a profound exploration of the African American experience, delving into issues of race, social justice, and the quest for equality. His insight into the racial divide and its manifestations in American society was a call to acknowledge and address the pervasive problem of racism.