Final answer:
Martin Luther King Jr. became the most prominent symbol of the civil rights movement during the March on Washington for Jobs and Freedom in 1963, where he delivered his famous 'I Have a Dream' speech.
Step-by-step explanation:
The event that made Martin Luther King Jr. the most prominent symbol of the civil rights movement was the March on Washington for Jobs and Freedom in August 1963. MLK's delivery of his iconic "I Have a Dream" speech not only resonated with the nation but also served as a galvanizing moment for the civil rights movement. His vision and articulation of a future where racial injustice ended and harmony prevailed were key in establishing the legitimacy of the movement's goals. Although there were several significant events where MLK played a central role, such as the Montgomery Bus Boycott and the Birmingham campaign, it was the March on Washington that ultimately positioned him at the forefront of the struggle for civil rights.