Answer:
He uses the persuasive technique called "pathos".
Step-by-step explanation:
When Martin Luther King says "I had a dream", he is trying to gain public empathy. Empathy is a type of emotion, and King is making a speech with a strong appeal to the emotional of his audience, where he shows his desire for equality among all human beings, regardless of their color.
When a speech has a strong emotional appeal, we can say that this speech is using the persuasive technique called "pathos" which, according to aristoles, means "suffering and experience", where the speaker, through a subject in which he has a vast familiarity and experience, seeks to provoke its followers by appealing to their emotions.