The answer is a. Spinal cord.
The two main parts of the central nervous system are brain and spinal cord. Cerebellum, cerebrum and brainstem are the part of the brain, leaving the spinal cord as the right answer. The spinal cord serves as the main communication link between the body and the brain. It is made of 31 segments containing both sensory and motor nerve roots. These nerves connect it with muscles, skin, and other peripheral tissue and organs. The spinal cord receives information from the body and relays it to the brain, and vice versa, it receives information from the brain and relays it out to the effectors.