Of the options that you have given us, the best is A. Answers B and D are simply wrong. Answer C is not really correct either even though the Nation of Islam would have generally supported the idea of Black Power. Although they would have supported it, it was not they who came up with the idea.
Black Power was an idea that was popularized by Stokely Carmichael. The idea of Black Power was that efforts (such as those of Martin Luther King, Jr) to work with whites were misguided. Carmichael believed that blacks should work on their own, without help, to demand their rights. He thought that anything else would lead to black people continuing to rely on whites to give them rights, instead of simply taking those rights for themselves. He also believed that King's tactics would result in less pride among Black People.
For these reasons, A is the best answer.