Which part makes up the bulk of the brain?

• A. Cerebrum
• B. Cerebellum
• C. Brainstem
• D. Diencephalon

Answer: (A)

The part that forms the bulk of the brain is the cerebrum. It’s the largest region, making up roughly 80% of brain weight and consisting of the two cerebral hemispheres and the outer cerebral cortex with the underlying structures. This area is where higher functions live—thought, perception, memory, sensation, and voluntary movement—so its size reflects its broad range of roles.

The other regions serve important but smaller roles: the cerebellum sits under the back of the brain and mainly coordinates movement and balance; the brainstem connects to the spinal cord and manages basic life-sustaining functions like breathing and heart rate; the diencephalon (containing structures such as the thalamus and hypothalamus) handles sensory relay and autonomic/hormonal regulation. Because the cerebrum occupies most of the brain’s mass and handles the majority of complex functions, it is the correct answer.