Answer: I would contend that the right answer is the C) "like startled rattlesnakes, the long / naked arms uncoil."
Explanation: Just to elaborate a little bit on the answers, it can be added that the two halves of a castanet, which can be made out of wood or other material, are joined by a cord that usually goes around the thumb, while the other fingers tap the surface, creating a very characteristic and cheerful sound. In the poem, the speaker compares this sound and visual image of the dancer playing the castanets to surprised rattlesnakes that unfold in an (usually frantic) movement.