The correct answer is the following: the belief that dreams often remain unrealized due to oppression.
Actually in the fourth four line unit the note of skepticism is difficult to understand infact he talks about his world dream where misery doesn’t exist in men’s lives because it has replaced by the Joy. Finally Hughes comments that a world like this can only exist in his dream vision.
In “I Dream a World” the character is the dreamer and visionary in the scheme of things but also the reader is engaged in this dreaming process, we can say that both dream the same dream. The implicit function of every four line unit is to explain the reality that the vision of Hughes’s dream try to resolve. Hughes’ contrasting versions of the world or as he imagines are presented in each unit too.