Final answer:
Martin Luther King joined the Montgomery bus boycott in December 1955 after Rosa Parks' arrest. The boycott lasted for 381 days and aimed to protest racial segregation on the public transit system.
Step-by-step explanation:
Martin Luther King joined the Montgomery bus boycott in December 1955 after Rosa Parks was arrested for refusing to give up her seat to a white person. The boycott lasted for 381 days, during which Black Montgomery residents organized carpools, paid for rides in African American-owned taxis, and walked to their destinations to protest the racial segregation on the public transit system.