The correct option is C It's about the restrictions resulting from racism and oppression.
This poem by Maya Angelou talks about the plain racial discrimination between the Blacks and the Whites. It presents to us the oppression that the Blacks were subjected to and reminds the reader of the long abolished slavery: this is best presented by Angelou's use of a "free bird" and a "caged bird". In this context, option B is the best answer.