The correct answer is discrimination against minorities.
Martin Luther King Jr. was an enormous part of the Civil Rights movement during the 1950's and 1960's. The Civil Rights movement was focused on helping African-Americans gain equal rights in the United States. Martin Luther King Jr. was heavily involved by giving speeches exposing the discrimination African-Americans faced on a daily basis, organizing marches on Washington to show the government that citizens were not willing to accept this unfair treatment, developing peaceful forms of protests to expose the unequal treatment facing minorities, etc.