15.We cannot be satisfied as long as a Negro in Mississippi cannot vote and a Negro in New York believes he has nothing for which to vote.No,no we are not satisfied,and we will not be satisfied until justice rolls down like waters and righteousness like a mighty stream.

This question is asking for the identification of the speaker of the given quote. The quote is a famous line from a speech by Dr. Martin Luther King Jr., a prominent leader in the Civil Rights Movement in the United States. The speech is known as "I Have a Dream" and was delivered during the March on Washington for Jobs and Freedom on August 28, 1963. The quote reflects the deep-seated frustrations and aspirations of African Americans during the Civil Rights Movement, emphasizing the demand for justice and equality. Dr. King's speech is one of the most iconic moments in American history, symbolizing the fight against racial injustice and the quest for civil rights. The imagery of "justice rolling down like waters and righteousness like a mighty stream" is a poetic expression of the hope for widespread and enduring justice and moral righteousness. This speech and its famous line have since become symbols of the broader struggle for civil rights and social justice in the United States.