The best of these answers is C. The assassination of Dr. King and the leadership vacuum that it left was a major factor in the demise of the civil rights movement.
Over the years, Martin Luther King, Jr. became the face of the civil rights movement. Whites came to identify him as the leader of the movement and came, in many cases, to trust him. Many blacks looked up to him as the most effective leader that they had. No other figure in the black community had as much credibility with blacks and whites. Therefore, when King was killed, the movement was hit very hard. There was no longer a leader who could keep it together and, at the same time, get white support.
However, I would argue that the movement was declining anyway and that King's death only hastened that end. All the goals that white America could really agree with had been achieved and the movement was going to have a hard time moving forward.
But that is not one of your options so the best answer is C.
In support of Pohnpei the best answer in this case is "C" reason being;
The civil rights movements had made considerable achievements across the United States and especially in the South where racial discrimination was rampant. Several matches had been conducted led by prominent activists such as Martin Luther King Jr. among others. Spearheading organizations were also established to guide the process such as the NAACP, SCLC and SNCC. The protests were predominantly guided by the principle of non-violence, however most participants were injured during confrontations with law enforcement and many of them were arrested. This led to the rise of militant groups who viewed the non-violent means as weak. Groups such as the Black Panther party brought about a myriad of challenges especially on the issue of guiding principles of the older organizations such as the NAACP. The assassination of Martin Luther King Jr. and other notable luminaries led to the near collapse of the institutions; this further precipitated infighting among the different groups. Apart from this, government suppression brought continued challenges to organizers and eventually this weakened the protest activities in the late 60s.