The correct answer is d) cerebellum.
The brain stem is composed of three primary components: the midbrain, pons, and medulla oblongata. Each of these structures plays a critical role in regulating vital functions and connecting different parts of the nervous system. The brain stem is responsible for essential life functions such as breathing, heartbeat, and blood pressure.
On the other hand, the cerebellum, while an important part of the brain, is not part of the brain stem. It is located at the back of the brain and is primarily involved in coordinating voluntary movements, balance, and motor learning. This functional distinction is why the cerebellum does not belong to the brain stem components.