Answer:
A). He knows that many great leaders in history were extremists.
Step-by-step explanation:
In the context of 'Letter from Birmingham Jail,' the key reason for which Jr. Martin Luther King feels dignity and contentment on being criticized as an extremist is that 'he knew several great leaders who've brought change and reform in the society have been extremists.' Thus, he feels satisfied with being called an extremist as he knew he was on the right path in his fight against the oppression and injustices on the blacks. Thus, option A is the correct answer.