The statement that best explains how the word "painful" impacts the tone of the speech is "It helps create a concerned, empathetic tone".
When Martin Luther King talks about "painful experience" in "A Letter from Birmingham Jail", he refers to all the people that were persecuted by the oppressor for standing up for their rights and fighting for freedom. The tone of the speech is concerned and empathetic because King, who was the leader of the Civil Rights Movement, feels pity and sadness and is troubled for those who could not achieve their goal of building a fairer and more equal society.