Answer:
C). 'Symphony of brotherhood.'
Explanation:
As per the question, the phrase that connotes to back King's key purpose to motivate hope by reflecting the 'beauty or belle of an equal society' would be 'symphony of brotherhood' as the use of word like symphony emphasize his idea of an integrated nation. This phrase appropriately serve his purpose of 'transforming the jangling discords present in the nation into a beautiful symphony' which personifies their unity and strength. King always desired to have a nation where people would be able to 'work together, to pray together, to struggle together, to go to jail together, to stand up for freedom together' and possesses a hope to stand united as a nation. Therefore, option C is the correct answer.
