The correct answer is D) Regional.
Studying how the midbrain, a specific structure within the brainstem, works would involve examining its anatomy, physiology, and function in relation to the broader nervous system. This is a regional approach as it focuses on the specific region or structure within the nervous system. Options A) Molecular, B) Systems, and C) Cellular would involve studying different aspects of the nervous system at a different level of organization.
I hope this helps.