Final answer:
The rhetorical technique used in paragraph 3 of Martin Luther King Jr.'s "I Have a Dream" speech is parallel structure, as noted by the repeated phrase "One hundred years later,".
Step-by-step explanation:
In paragraph 3 of Martin Luther King Jr.'s "I Have a Dream" speech, the rhetorical technique used to add emphasis is C) parallel structure. Parallel structure involves using the same pattern of words to show that two or more ideas have the same level of importance. This can be observed in the repeated use of the phrase "One hundred years later," which emphasizes the ongoing struggles faced by African Americans despite the passing of a century since the Emancipation Proclamation.