Answer:
D. The black citizens of Montgomery refusing to ride city buses
Step-by-step explanation:
The Supreme Court ban on segregation on public transportation was a result of the Montgomery bus boycott, which was a civil rights protest during which black citizens of Montgomery, Alabama refused to ride city buses in order to protest the discriminatory treatment they received on public transportation. The boycott began in December 1955 and lasted for over a year, leading to a Supreme Court ruling in November 1956 that declared segregation on public buses to be unconstitutional. The boycott was a major event in the Civil Rights Movement and helped to bring national attention to the issue of racial segregation and discrimination in the United States.