The line that supports Douglass's claim that the Fourth of July is not a cause worthy of celebration by all is:
C. Fellow-citizens, above your national, tumultuous joy, I hear the mournful wail of millions! whose chains, heavy and grievous yesterday, are, to-day, rendered more intolerable by the jubilee shouts that reach them.
This line emphasizes the contrast between the celebratory mood of the Fourth of July and the suffering of enslaved people, highlighting that for many, this day is a reminder of their oppression rather than a cause for celebration.