The best answer is:
B. by providing in the subsequent paragraphs the successes of the Montgomery Bus Boycott
The author details the events surrounding the boycott, including the impact of Rosa Parks' arrest, the organization of the boycott by Dr. Martin Luther King, Jr., and ultimately, the successful desegregation of the Montgomery buses. These points collectively illustrate the success and significance of the boycott, demonstrating that it was not a "fruitless endeavor."