Final answer:
The most primitive part of the CNS is the brainstem, which controls vital functions and communicates between the cerebrum and the spinal cord. The correct option is b) Brainstem.
Step-by-step explanation:
The most primitive part of the CNS (Central Nervous System) is the brainstem. The cerebrum is indeed a large component of the CNS and stands out due to its folded surface known as the cerebral cortex; it is responsible for more advanced brain functions such as thinking and processing sensory information.
The cerebellum coordinates voluntary movements and balance, while the diencephalon acts as a relay center facilitating the communication between the cerebrum and the rest of the nervous system.
However, it is the brainstem that controls basic body functions like breathing, heartbeat, and digestion, and it also carries information between the cerebrum and the spinal cord, making it the most fundamental structure of the CNS. The correct option is b) Brainstem.