Final answer:
Martin Luther King, Jr. gained national prominence during the Montgomery bus boycott of 1955, which was a key event in the American Civil Rights Movement.
Step-by-step explanation:
Martin Luther King, Jr. first came to national prominence during the Montgomery bus boycott of 1955. This pivotal event of the Civil Rights Movement followed the arrest of Rosa Parks for refusing to give up her bus seat to a white person. Dr. King's leadership during the boycott, which lasted over a year, illustrated the power of nonviolent protest and eventually led to the U.S. Supreme Court declaring bus segregation unconstitutional in response to a lawsuit filed by the boycotters. Dr. King's involvement in the boycott marked his rise as a central figure in the struggle for civil rights, as his nonviolent approach garnered both national attention and respect from government elites.