Correct answer: a campaign to register African American voters.
The campaign launched in the summer of 1964 by the Congress of Racial Equality (CORE), in cooperation with the Student Nonviolent Coordinating Committee (SNCC) and the National Association for the Advancement of Colored People (NAACP), was called "Freedom Summer." The aim of the campaign was to get African Americans registered to vote. The efforts focused especially on the state of Mississippi, where up until that point less than 10% of African American citizens were registered as voters.
As to the other answers, the Birmingham Campaign, in Alabama in 1963, was led primarily by the Southern Christian Leadership Conference (SCLC) rather than by CORE, and included the Chidren's Crusade.