The correct answer is option D, that is, brainstem.
The innermost region of the brain is known as the brainstem, which continues with the spinal cord. It is the part of the brain, which associates the cerebrum with the spinal cord. It comprises the medulla oblongata, pons, and the midbrain. The sensory and the motor neurons pass via the brainstem permitting for the transmission of signals between the spinal cord and the brain.