Based on their views and actions described in the passage, the most fitting answer is:
B. Robert likely joined the Southern Christian Leadership Conference and Carol likely joined the Student Non-Violent Coordinating Committee.
This is because Robert's focus on organizing letter-writing campaigns and marches aligns well with the Southern Christian Leadership Conference (SCLC), which was known for its organized protests and advocacy, often led by church leaders. Carol's wish to participate in sit-ins reflects the activities of the Student Non-Violent Coordinating Committee (SNCC), which played a significant role in sit-ins and other direct action protests during the civil rights movement.