The central nervous system is:
a. all portions of the nervous system that are not encased in bone
b. made up of the brain and spinal chord
c. the portion of the nervous system that controls the beating of the heart and other "central" organs
d. covered in a myelin sheath

b. made up of the brain and spinal cord

Explanation:

The nervous system has two different parts:

The Central Nervous system which consists of the brain and the spinal cord and its function is to integrate the information gathered from all the other parts of the body.The peripheral nervous system which consists of the nerves that branch off from the spinal cord and that go to all the rest of our body. It's function is to gather the information from the different parts of our body and send it to the Central Nervous system.

Therefore, the Central Nervous System is b. made up of the brain and spinal cord