Answer:
D. Anaphora
Step-by-step explanation:
An anaphora refers to a figure of speech that characterizes for repeating words at the beginning of succesive clauses, phrases, or sentences. This in order to make an artistic affect. So Dr. King repeats the phrase in order to emphasize that he is seeking for freedom, this in an artistic and memorable way, having a deeper impact on people that will also remeber his speech.